Look forward to the photos.
Just so you know, Coyote, Fox, etc. will hunt in the day and take opportunity. I lost a cockerel earlier this year from Coyote during the day. One the previous year to Fox during the day. They are quite sneaky. But having a secured night area is good too!
AE had crossed my mind early on, but when I refreshed my memory of what I had read about it, I sort of canceled that avenue because of age and leg tremors.
Some neurological symptoms can be hard to nail down - there's SO many causes and conditions - from genetics, incubation issues, myco and...
I get it! Sometimes (especially in today's retail market) you have to go with what you can find!
For the vitamins, I just pop the pills (one at a time) in the beak and let them swallow. Easiest way I know of doing it.
The new grower is medicated? Is it chick starter with Amprolium? At their age, they likely don't need medicated feed. It won't hurt them, but they should have already built resistance to the Coccidia strain(s) found in their environment by now.
I think you mentioned you found his crop, but...
Is his crop emptying overnight?
Any swelling of the leg joints?
What's his poop like?
Lice/mites?
Is flax seed something you normally feed your chickens?
Photos of him and his poop may be helpful.
What protein % is the grower vs the chick starter?