I've personally ran a few FOSS games on Deck. Typically if you want them to work automatically in terms of controls you have to add them to Steam from Desktop Mode and then go to the community controller configuration. MOST games from the FOSS community I've tried had some community bindings, though keep in mind that for some more obscure ones you may have to do some heavy lifting yourself and make a control scheme. It's also important to note that Steam uses the name of the application as a means to connect it to Community Control Schemes, so if you change an applications name or if the application recently had an update and changes its name every update, IE: (Game 1.2) to (Game 1.3), Steam may lose the ability to tell what game it is and show none of the Community Control Schemes. Ones you've already downloaded will stay however.

I can't say I've personally ever done any dual booting on Deck, but I know that it is fairly straightforward and works quite well. The only thing to keep in mind is this is unsupported by Valve, so Game Mode's excellent sleep mode won't work and instead it will do a laptop style sleep mode. Other deck specific features may be broken as well.

In terms of patches, there is no super unified way to tell what's up to date. If you go to the Steam Deck website and go to the news section, you can read the patch notes. https://www.steamdeck.com/en/news I'm not super concerned with being on the bleeding edge for my use case with the Deck, but I have heard that recent updates have brought the deck to a much more recent version of Mesa for example.

One last thing to remind you as well, Steam OS that comes on the Deck has an immutable file system. (I assume that's why you are asking about other OS installs.) While you can get under the hood very easily, you should be aware that any OS update will wipe anything installed outside of the immutable file system, but you still will have readily available access to reinstall it once you set a root password for the first time.

Hopefully some of this info is useful!