It wasn't him. I wish(ed) it <were> <'d been> him.

[jutfrank]

2a. Anton said he couldn't help her and that he wished he could (help her). (said + past-past)
3a. Anton said he can't help her and that he wishes he could (help her). (said + present-present)

Yes, those are good.

This is a pretty complicated part of English grammar for me. I don't quite understand why "couldn't" keeps this possibility.
After all, 'couldn't' is a backshifted version of 'can't'. I was taught that in the reported speech "I can't help her" changes to "He said he couldn't help her" with the same pattern as "I want to go out" changes to "He said he wanted to go out".

Remember that can't can be about the present or the future. If you backshift it, you may still be talking about the future.

a) He said he can't help her.
b) He said he couldn't help her.

Whereas a) makes it clear that help is still needed, we can't tell from b) whether that's the case—it may still be needed and it may not.

c) He said he hadn't been able to help her.

The meaning here differs in that not being able to help her was relevant at a point prior to the time when he said it.