The one to the left of the pillar in Ridley. That one is easy enough to get.

Don't even bother fighting Imago. He's not required for 15%.

It's not really an original console vs. emulator discussion. The WiiU VC has savestates which are banned for example, and similarly I could create a controller that has a button macro which would also be banned. Binding down to two different buttons seems fine to me though even if it doesn't work as well as you described on the gameboy player. If I press down on my gamecube controller and then on my SP for example it only registers one down input.

This would definitely be against the rules. Allowing something like this would open a huge can of worms.

Just about everyone who records a console setup records from a gameboy player. You can definitely use one.

Using emulator functions before the start of the run (by making a savestate and using it right before loading a save as an example) is now banned. Your run must be absolutely free of emulator specific functions after you do a soft or hard reset. This was never specifically banned before, but it was definitely discouraged. If you have any questions feel free to post them, but I think that this is a common sense rule.

I think that there's a bit of a delay when doing that with the GBP. This is something I'd have to look into though.

I am a little weary of mapping an input to multiple buttons since it's an emulator only function, but I can't really think of many ways to abuse it, so it would likely not get banned. The best I can think of is being able to map two buttons to down so that we can morph faster. It is probably easier to do a heal glitch setup with this.