That feature is not supported right now, because RetroX is using an older version of RetroArch, sorry

You need to have the game already installed.
A quick way to get the installed files is to run the installer in a PC, it will create a folder, then use that created folder.

Right now RetroX is using snes9x_next_libretro

It depends.. if the bundled emulators are Standalone emulators, I'm stopping using them for their complexity on maintaining them working with time, I'm switching to RetroArch cores from some time ago. If the emulator that works good for you is a retroarch core, you can select them in the game option...

Please check these docs: http://retrox.tv/artwork

About N64: I think I can do what you propose, it's a similar situation we have with Dreamcast right now, where some games are tolerant to work on non-stretched 16:9.

About shaders per system: Yes, that feature is needed as well

So it is like a shortcut to the global settings instead of a in-game setting, correct?, I imagine you would need to make it work separately from the global settings. Yes, that's correct. Yes, I know, but still, it affects all N64 games instead of the game opened in the background, which is the only...

Those are different settings: The first settings that you post are global settings, so you can switch resolutions quickly before starting the game without going to the global settings menu, those apply for all emulators except.... The settings for N64 that you send later titled "Detalles del ju...

From what I can see in that page, Reicast standalone supports only v4, and the libretro core support v5. Right now RetroX is using the standalone version. Now, that will change in the "near" future to use the libretro core (flycast), so I wouldn't recommend you to change the format to all ...

It cannot be done unless I break compatibility with older devices. (Thanks Android)
If I find a way to do it without having that problem I'll make the change