Any feedback would be warmly welcome. I feel like we're at a critical point and don't want to caulk this up.
Feedback needed on eclectus harness training technique - YouTube
First off, I think it is great to show the harness first up and treat for a calm reaction, so good job

Also great ending on a 'good' note.
The main issue though is you are placing the harness over him, and Parker is reacting negatively to that, by moving away from it. ( I know this has already been mentioned and explained and you have responded, but I'll put my .02c in :09

I also personally don't see the point in using the wrong hole to start with. You will have to re-train when you are trying to fit his head through the much smaller head hole so it seems a bit like a waste of time. Don't get me wrong, I can see why you are doing it, I just think it is an unnecessary step and I think he will find the head hole to be so completely different that the original training will not have helped him much.
My suggestion is to hold the harness with one hand, with your fingers spreading open the head hole and pushing as much of the harness away from the head hole, just so Parker has a clear sight of the hole he needs to get through and the opening is wide enough so he can stick his head through. The other hand will be used to hold the treat.
Hold the treat behind the head hole and wait for HIM to move closer. Don't move the harness towards him, ever. Make him do the work.
First hold the treat through and in front of the opening so he doesn't have to put his head through. Then the treat will be at the opening, then behind it, so he understands, 'Oh ok, my head has to go through that thing". (You have mentioned he was already passed this step so that is good)
Do not move the harness towards him at all. He needs to come to it. In no time, he should be voluntarily putting his head through the hole. This to me is a lot better situation. He is making the choice that it is OK to put his head in there, you are not forcing it over him and making him uncomfortable. Release the treat when his head is through the hole.
Work on that for an entire session so he is comfortable. As soon as he gets the treat, he will most likely take his head out. It's OK he took his head out, but we don't want to reward that, so no treats, no comments no 'good boy'.
At this point, you are not dropping the harness on his body... He is putting his head in, taking it out, without the harness on him. Only when he is good with that do you move forward.
Next step is getting his head through, and 'dropping' the harness down over his neck. Hold off your treat now until the harness is sitting over his neck. No more treats simply for sticking his head in the hole! Gotta raise criteria. If he struggles at this point you have gone too fast.
Again, he will likely take his head out as soon as he gets the treat. This is OK, just don't comment on it or reward. Try and give the treat then remove the harness, though. That is all you are asking for at the moment, don't move forward too quickly. He must be comfortable before you start asking him to keep it on him for extended periods.
When he is reliably putting his own head in the harness and you can drop it on his neck, say a command for it
the moment his head is going in the harness. Not before, not after. He needs to understand the command is related to the action of him putting his head through the harness. (Much like when you are training a dog to sit, you don't link the command word with him lowering his butt, but with the moment his butt touches the ground) My command is 'Harness'. Commands are important

When he learns the word with the action, you will be able to say it right at the beginning and he will know, 'Ok, I put my head in now'.
Next step, when he starts getting comfortable (you should be able to tell), you want to keep his head in the harness, by giving him lots of treats one after another while he is staying there (Jackpot!!), Because he is distracted by lots of treats in a row, he should keep his head in there.
Try and remove the harness BEFORE he removes his head from it. It's not a big deal if he takes his head out, but don't reward it, don't comment.
Now I think an important step here, that I don't see you doing, is giving a release command. Release commands are very important! Think of it like this: You can train a dog to stay, but if you don't give a release command, the dog will just break the stay whenever he wants, instead of waiting for you to say 'OK' before he is allowed to move. They have to know when the behavior is over, otherwise they just get confused with what you are asking.
You will introduce the release command when YOU are removing the harness from his head (Not when he is taking his head out of the harness). My release is 'Take it off!" Then when you take the harness off with the command, he can have lots of treats afterwards. If there is no command and you give him treats, he is just getting treats for not having the harness on anymore, which is counter productive.
I would spend a lot of time on this step before moving forward as it is important.
Eventually you should be able to lower your treats and he should keep his head in the harness until you release him to take it off. Ideally you will get to a point when he doesn't receive a treat until the entire behavior is completed.
That is, you say the command -
Harness, he walks towards it, puts his head in, you dropping the harness down his neck, you wait a few seconds, you saying your release 'Take it off' and him taking it off = Treats!!
That would be the goal before you move forward to the wing loops. He needs a clear understanding off all the elements of putting the harness on before you move forward to the next big thing. I would say the behavior is broken up into 3 big steps. 1) Getting the head loop on and sitting with it on without taking it off. 2) Getting the wings through the loops without fussing 3) Tightening the harness.
Also, you can surprise him every now and then by ending the behavior as soon as he attempts to put his head in right at the beginning and giving him a treat. That is nice he is offering the behavior so why not reward him for it every now and then? The best time (Or, only time) to randomly treat here is when he is being particularly enthusiastic or quick at offering the behavior. We want that! We don't want a hesitant bird who takes his time. We want to reward eagerness!! The faster, the better!
I'm currently re-training my boy with the harness after a very long absence with it. (He is 2.5 years old, has gone without harness for 1 year)
When I first got it when he was young, I didn't train properly. Yes, I was one of those people who was like, "Oh look, my baby bird is doing just fine here. I don't need to do the serious training, he can get it on with no trouble!" Ha!
He would 'accept' the harness but I had to do all the work, often hold him while doing it because he kept trying to avoid or fly away, sometimes he would bite as well. So I'm starting fresh with him right from the beginning and I'm currently on training session 3 and at this step, where he is holding his head in the harness for some time and I am introducing the release command.
So, you can take my advice with a grain of salt given I don't have a harness trained bird yet, but it is working and I'm familiar with clicker training for dogs so I understand the importance of getting the animal to do all the work
