1) For this yes, you would create one child profile for each side, with the UPC belonging to the parent. If the disc ID clashes with another, earlier release of the same disc ID, then you can still link it to the parent as a child, just not work on the profile itself to customise it as a child to your set. Well, not for the online DB, at any rate.

2) Not much that can be done about this. Best best is to stick all the details in the profile (combine running times, add features from both films, use the production year of the earliest film etc.) and divide the credits using headers.

3) Much the same as #1, really, one child per disc instead of per side. The overview should be the entire relevant part of the back cover for the parent, if there is a separate description for each movie then only put the relevant section in the child. If the overview is only a brief outline of both films, it might be more useful to invoke the original overview rule, which could involve borrowing from a single release, yes. Although as with #1, if the disc ID clashes with an earlier release, the child can only be linked, not edited for the online DB.

Yes, you'd definitely create a child for each movie via disc ID. You can certainly use the standalone release as a template for your child profile, and then check the details against your disc. The image would be from your Action Double Feature release, which would also be the source of the DVD release date and the case type.