I admire your relationship with Abi, Donna that little bird loves you, there is simply no other way to say it.
I wish I knew what has happened to Abi, its a world of speculation, but something has happened, your gut feelings where correct, I am very pleased that Abi wasn't harmed real bad, but what shines is no matter what has happened, Abi has come back to the person he trusts...YOU.
The tail feathers are the birds steering wheel, Abi will have a hard time turning, and landing correctly, this will place him more at risk when it comes to escaping a preditor.
Please don't concern yourself about the box trap you came up with, I knew I had posted the trap somewhere on the forum before, I was very tired, been trying to get my yard work caught up, after working all night long this man needs rest not more work...

.
In my opinion, Abi needs to be contained, the feathers will grow back in, this will take time, the shed will work fine, do your best to earn the trust that will be lost during containment, Abi will understand over time why you are caging him, when you feel all is okay, then set Abi free...
Here's your chance to give Abi a kiss and hold him close to your heart.
When all is done, No, I don't think Abi will abandon you, your relationship is to strong.
Oh he may fly off when released, but his love for you will bring him back.
Catch him, hold him close, give him a kiss for me, then allow him to fly in the shed, be careful when entering at exiting the shed, he will try to flee at the slightest chance, be quick about entering and exiting...
The trust, the love...is based & built from AFAR, oneday Abi will allow you to be closer, oneday maybe even a touch, but this should be up to Abi.
For now, its up to you, Abi must be touched and handled, you will only be protecting him until all is back to normal.
Good Luck, My prayers are with you both.
Keep us informed.
Arty.