Yes, you could put this in functions.php
The format shouldn’t matter. ACF stores the attachment ID as the field value no matter what you set it up do return.
Not sure, it looks like it should work. I will see if I can find some time to test it. Basically we car calling our function whenever the post is updated, grabbing the featured image id and stuffing it into the ACF field which is kinda why I’m confused as to why it doesn’t work (even though I didn’t test it before I posted it)
