Yes but I wasn't too comfortable with hacking the remote control because I wasn't sure if the button shorted the two contacts towards Vcc or ground (so if I needed an NMOS or PMOS). My knowledge of electronics is super minimal, I can read schematics, design simple circuits, size resistors, etc. but reverse engineering is beyond my level.

Also I used this remote control only because one of the switches was broken, I didn't have a spare one so I couldn't afford breaking it.

An optocoupler would be better but I figured maybe one day I will repurpose the relay shields for something else, or unsolder the relays. Actually my next project involves both optocouplers (for efficient 12V->3.3V conversion) and relays to drive AC but I will probably have a PCB manufactured for that one!

Yes, though you will need to determine whether it should be pull up or pull down, and to what voltage level. And make sure not to provide it with too high a load (too low resistance), or too high a voltage, which may damage the circuit. Since the voltage levels might be different from your microcontroller, so I would probably recommend an optocoupler to simplify the interfacing. Gives the isolation of a relay but without the mechanical operation and size of one.

Yep, should work fine since you only need current flow in one direction!