I can’t give you concrete answers to your questions at the moment because I am still finding my way around issues like these, but it’s good that you bring them up. Part of my job is to document applications development on the Librem 5, so the corresponding mechanism to the application lifecycle on Android is something I’m aiming to describe.
Earlier phone environments like Qtopia had ways to effectively suspend applications, though this wasn’t based around a cross-platform standard. Again, things like writing applications that don’t just drain the battery is something that needs to be documented (see this issue) and taking advantage of existing frameworks to do this is desirable.