Returning Player/Thief
First off, welcome back! Let’s see how well I can answer these…
1) They are but part of it is knowing the encounter. I have a hard time meleeing bosses that I’ve not fought before and sometimes resort to shortbow if there’s a ton of people around (lag, screen clutter). Kessex is a good example of where at first I’d shortbow or P/P like crazy until eventually I figured out the bosses to where I could reliably backstab and survive.
2) I’d start here but be aware that it’s written more for end-game dungeon speedruns as opposed to general PvE. Still, it’s a good write up to give you ideas and direction: https://forum-en.gw2archive.eu/forum/professions/thief/Thief-PvE-guide-by-Xukk-Jeremy-Chase
3) I, honestly, haven’t done PvP in GW2 yet but if you look around the forums the general consensus is that non-thieves say they’re OP but thieves say they’re UP. Take that for what you will…
4) Mass murder one backstab at a time…