Birds need sunlight, which they won't get in a basement, so it will be important to have good full-spectrum lighting for a minimum number of hours each day. Also, birds are very sensitive to radon gas which is not uncommon in basements. Most states have free radon test kits, you expose them according to the directions and send them back for analysis. Many other gases are heavier than air, so a furnace in the basement can create enough fumes to kill a bird without you smelling a thing. It's why the miners used to take birds with them - the birds die before anyone else experiences symptoms. So it can be very hard to determine what's safe. At the minimum there should be a radon test, a carbon monoxide detector in the bird room, good HEPA air filters especially if you have oil heat and because dust settles to the lowest levels.
Every parront should plan for emergencies, and since you know how easily these can occur you can take steps in advance. Doors locked is a good thing, but you know those ordinary locks can be opened from outside. If it's a real lock, some one needs to have a key in case of emergency, fire, etc. happens when you aren't home. And it might be hard to keep people from finding the key...not much you can do about that. If you believe that someone would really harm your flock, then you really don't have a way to guarantee their safety.
A good first step is to assemble an avian first aid kit. Get a good first aid manual for parrots - maybe someone here has a recommendation, or at least go online and print some things out to keep in a binder. Find a vet, preferably a certified avian vet, and call them. Find out what the hours are, what to do after hours, and best of all see about bringing them all at once for an introductory well-bird check. That shouldn't be very expensive, and it will get you established as a patient in the event something happens - they will have medical records on hand. The vet visit would be a great time to ask the vet what kind of emergency supplies and reference books to have. Emergency supplies might even be something like bird formula or Pedialyte for hydration etc.
You will find it helpful to set up a separate savings account for the birds. Figure out what they need for food, toys, light bulbs, cuttlebones, basic items and add in the cost of at least one well bird check a year for everybody, then set up a savings schedule for yourself until the account is fully funded for a year. Keep putting money into it regularly, and then if there ever is a big, middle of the night emergency you'll know you can get them the help they need.
Start getting parrot books from the library. Most libraries will get them for you on inter-library loan if they don have it. Read all you can. When you find a book you especially like, look for a copy on eBay or Amazon used books, Alibris. I find parrot books in used bookstores. They aren't all good, especially some of the older ones, so put together a list of recommendations from people here. There may already be a thread for that.
Before you bring any birds back home, ask yourself: how will I know if something is still wrong? Write up an agreement between you and your birds and sign it. You might say, for example, that if there is a single attempt to harm them in any way that you will immediately remove them from the home. So that means there must be a backup plan - where, who, how. If a bird escapes the house, gets hurt in the kitchen, gets attacked by an animal, any of these things is not just a warning, it's an emergency full stop, put the contingency plan into effect. You must swear in your heart and your soul that you will protect them, and then be faithful to that oath.
The funny thing is, this the same advice I'd give to anyone thinking of getting a bird. How will it live, how will it be cared for, how will it get vet care, what does it need to eat, what kind of attention, how can every danger be anticipated and defended against, and what will happen if the bird needs a new home. I'm old, much older than Gus, so I assume he will outlive both me and my husband and we are planning for him. Part of that is making sure to leave money for his care, and part is finding the right person to either take him or to find him a good home and make sure he's being treated well. So you see, none of this is personal, except that you know some of the things that happened in the past and must be guarded against.
Think of bringing them home as if this were a new thing, a decision being made for the first time, and be sure that you will be able to give them a better, safer life than anything else they would have out there. It's a lot to think about and do, so it might be helpful to start making a written plan with checklists, and then execute that plan as earnestly as you've ever done anything in your life. That's my advice, for what an old fart's advice is worth. Oh, and also, be sure to treat yourself with kindness and love, because if you don't love yourself you won't be able to love any other being. If you make your decisions out of loving kindness you may not always be right, but you will never be wrong.
