Apps from the Mac App Store may be used on any Macs that you own or control for your personal use.
2) Difficult to say what will happen in the future. Historically these updates have usually been free (last paid one I recall was 7.2), but Apple did just make the Mainstage 2.2 update a paid one.
Normally, updates via the AppStore are available free (but you might want to backup the old version first, in case of problems)
3) see 1)