Piggybacking on this as Im also interested in the Touchpad Stuff -- curious if theres a method to access the left and right touchpad inputs?
Ill try to answer what i can of the other questions:
1. Technically yes. I've actually found the Greengrinds plugin to have better support for Steam API tho. construct.net/en/make-games/addons/244/greengrinds
2. Steam Deck has an interface to close games. (use the steam button to bring up a menu, then hit "Close Game"). Most games typically have a "Quit Game" option on the main menu though. I think its preferred especially if people want to go full screen. Games always launch full screen if youre in the Game mode (not desktop mode). So typically games with a "Full Screen" option will just.. do nothing when you enable/disable it. Still good to have for people playing on desktop or in desktop mode of steam deck.
3. Yeah I know greengrinds for sure has an "Is on SteamDeck" event condition. I think the official plugin does as well.
4. I think Browser-Close is best. tie that to an in-game Quit button and youre good!
5. Steam Deck has its own unique button for bringing up steam menus / overlays. So just use whatever button you feel is best -- it wont conflict with default steam deck inputs! I think you WILL need to map the gamepad start button to do what you want it to do. Basically if you connect a Xbox / Playstation controller and setup your game to work for the gamepad, the gamepad settings should carry over to the steamdeck 1:1. So you can basically use a gamepad to test steam deck functionality.
6. this is what im stuck on too. ill try some stuff this week and let you know if i learn anything, but happy to get any info if anyone else has the knowledge already available 😃