Honestly, after reading through this entire thread, it seems to me that Sammy's behavior is most likely being caused by hormones. Just because he was acting hormonal prior to his aggressive behavior towards people doesn't mean that the hormones themselves haven't progressed, thus progressing his behavior. You said that he started to slightly lighten up a bit when you were starting to "decrease his daylight hours", so I think that was your cue to what the problem is. If he humps things several times a day and is searching for nesting spots, and has been doing this for years as you said, I'm almost certain your issue is his hormones are out of control. Some birds (as well as other pets like dogs, cats, reptiles, etc.), and people for that matter, are more effected by hormonal changes than others, some aren't effected at all, and some are just overcome by them. No one's systems are exactly the same, and while we use the generic terms "hormones" or "hormonal behavior", there is actually so much more involved and effected by sex hormones. It involves the reproductive system, the endocrine system, the brain chemistry such as serotonin and dopamine being effected by the primary sex hormones, the non sex hormone producing glands being driven for multiple reasons to also suddenly produce more primary sex hormones, such as the adrenal glands on the kidneys, it's just amazing what all is involved and effected by testosterone, estrogen, progesterone, or a lack there of.
My first question is was Sammy ever DNA sexed? Are you absolutely, 100% certain that Sammy is a male? And I don't mean "I'm certain Sammy is a male because he's very vocal and acts like a male, he's never laid an egg, etc.", I mean has he been DNA sexed? The fact that Sammy has never laid an egg means nothing, as a female that is physically unable to lay eggs can have this caused by the same reasons that can cause the aggressive, violent behavior as well. It is a very important that you know this for sure, 100% sure, so if you've never had this done or haven't been given a lab report proving it, you need to spend the $25 and have this done ASAP. I don't know how long you've had Sammy or if you are the original owner, and I've known many birds that were rehomed by their original owner and the new owner was told the bird was "definitely" a certain sex, and they were totally wrong. This can tell us a lot.
Regardless of Sammy's sex, you need to approach his behavioral issues not like they are behavioral issues at all, but rather physical, medical/health issues that are causing a behavioral change in his personality. I know your vet (I'm assuming this is a certified avian vet?) has said Sammy is "completely healthy", I hope that is based on fecal smears, throat, nostril, and crop swabs, cultures, blood work results, etc. and not simply on a physical exam, by the way, because if Sammy has not had these tests done in the last 6 months you need to get them done ASAP as well as the DNA sexing. It's crazy but something as small as a crop infection, gastrointestinal infection, respiratory infection, a parasite, etc. can cause exactly what is happening.
First of all, I think you said that Sammy's cage is in your main living room already. I don't know what you and your wife's schedule is like, but you need to get Sammy's daily schedule corresponding with the sunrise and the sunset, and it must absolutely stay this way year-round for this to work. His cage needs to be in a place that he can at least see subtle light changes in the morning at sunrise and in the evening at sunset. I suggest using a cover for his cage because he is in the living room, so if you guys watch TV or whatever in the living room after sunrise Sammy will still be able to sleep. Trust me, once you cover them they sleep, the noise and light from a TV or whatever doesn't bother them. The cover you use must be a bit translucent so that Sammy will be able to see the light change at sunrise. The alternative method to this is to relocate Sammy's cage to a different room, with at least one window and a door that shuts, and leave him uncovered. If you do this then he will wake up when he sees the sunrise and you'll give him his dinner just before the sun starts to set, and once you've said your goodnights you shut the door to the room right before the sun completely sets. If you keep him in the living room then he'll see the sun start to rise through the cover, or an even better way is if you can put the back of his cage only facing the window. I actually do this and it's awesome, works wonderfully well! I have my bird's cages covered on 3 sides only, the front and both sides are completely blacked out with a very dark cover, but the back of their cages is only covered with a tiny bit of overhang from the cover. This way I feed them in their cages at the very start of the sunset, and just before the sun completely sets I cover the 3 sides of their cages that are facing the living room, the back side of their cages facing the windows I leave mostly uncovered. I'm still in my living room the rest of the night, watching TV, playing guitar, playing with my dogs, whatever, and the birds are out! I do not lift the covers or talk to them once I cover them until sunrise the next morning. This method of covering only 3 sides and having the back against/facing the window makes my life a lot easier because I don't have to be up at sunrise the next morning, the bird will just naturally wake when the sun starts to rise, and once the sun is up they have ample light in their cage even if I'm not up to remove the cover right at sunrise. On work days I am up and actually remove the cover right as the sun rises and I've noticed that after doing this schedule for a while they will focus on the sunrise and watch it intently. Once the sun is completely up they start eating, playing with toys, talking, etc. On Sunday morning when I sleep later I get up and take the cover off of their cages asap, but they are already awake and watching out the window, usually they have been chatting away and playing since sunrise anyway. That's the easiest way.
You have to stick to this schedule strictly, they must see the sun rise and see the sun set, that's key to this working. So I get up at 7:00 in the morning on weekdays, and right now the sunrise starts here around 6:30 or so, but that back cage side being uncovered and facing the window causes them to wake up and they are already awake looking out the window when I uncover them. The sun starts to set here right now around 7:00, so I feed them around 6:00 right now, then cover the 3 sides of their cage as the sun starts to set right around 7:00, maybe a bit before. So they are getting a good 12 hours of sleep every single night, and their natural sleep cycles are now visually dictated by the sun. After a couple of weeks of having them on this "solar schedule" most people notice a drastic change in behavior, their bird gradually starts to become calmer, less aggressive, more affectionate, less humping, less nesting behaviors. You're essentially knocking them out of breeding mode, thus lowering the release of primary sex hormones into their bloodstream, lowering their levels. You're also increasing their amount of daily sleep (and R.E.M. sleep, or restful sleep).
Now if you do this for at least a month and you follow it strictly, and you see no change in Sammy's behavior at all, including less humping his perch, looking for places to nest, etc., then my recommendation (which some people try first because they're desperate, I rather try natural remedies before medical ones, but sometimes it's just necessary) is to talk to your avian vet about getting Sammy on injections of Lupron. Lupron decreases gonadotropin release, thus drastically lowering or completely stopping the production of testosterone and estrogen. It essentially stops all primary sex hormone production, so in females it completely stops ovulation and essentially causes a chemical menopause, and in males it halts most testosterone production (and estrogen production), thus stopping all hormonal, sexual urges and behavior.
Some people are hesitant to try Lupron with their bird, but if you speak to a certified avian vet that is familiar with its use in veterinary medicine, they will tell you that it is not only safe, it has saved many, many birds from being rehomed or ending up in a rescue. The Lupron injection lasts for either 3 months or 6 months, I'm not sure which is most commonly used in avian medicine, but either way you only have to get your bird the shot 2 or 4 times a year, it's not expensive, and you'll know within a month whether Sammy's behavior is being caused by hormones or not for sure. And if it doesn't help Sammy's behavior you just stop the injections. Keep in mind that if you do opt to try Lupron with Sammy, which honestly, in my opinion isn't a bad idea in your case because of how desperate you are, it will not change his behavior instantly. Just like putting him on a solar schedule takes 2-5 weeks to show results, Lupron can take a month or so to show outward results because of the amount of sex hormones already in Sammy's system when he gets the injection. So while Sammy will stop producing testosterone and estrogen directly after getting the first Lupron injection, it will take time for his body to metabolize the hormones that have already been released into his body.
There are always options, you are never completely out of them when it comes to helping your bird and trying to save your relationship with them. Sometimes you need some medicinal help, nothing wrong with that.
"Dance like nobody's watching..."
