1. They can actively resist! But it feels uncomfortable and leaves their god defenseless and might actually blight their god if the shinki realizes they are doing something wrong.

2. He's more than welcome to make any basic supplies like that. As a beginner, it would definitely be a low-level sort of potion, though, and work on small injuries and restoring a small amount of power.

3. I don't see why he wouldn't be able to make something like that if he was able to in canon! But he would have to figure out that it is something he could do first of all, seeing as he doesn't have memories of that at this point in time.