i will try my best to answer your questions but take into account i am a micro player and there will be better advice available from more advanced members.
1) i will usually look at their re steal stat if i get 3bet by the blinds...if the stat is low then i think our overall hands strength should be taken into consideration more. if the stat is high then we depending on our hand strength we could 4bet bluff for a fold or call and take it postflop..could then look at their postflop stats and see what their tendicies are..for instance if their cbet is big but their turn cbet is low we can float the flop and bet the turn....
2) CO V BTN could be that BTN has noticed you are stealing the blinds and he has come over the top to re take dead money if you dont play back..if you dont think he's capable of that then you would need to look at his stats and postflop tendencies...if his 3bet is low then you can fold out hands that are crushed pre and 4bet big hands as he will prob call. again it depends on the player you are up against....MP V BTN..the earlier postion we are in the less likely the BTN will be looking to win the hand pre flop and wanting you to fold..so depending on the villians range and stats you can either 4bet/flat or fold out a dominated range.
3) depends on villians range and the lines he took...sometimes its normal to go for thin value and sometimes you need a stronger hand than TPGK..just depends on your reads.
4) depends on the villian lol...theres alot of 'depends' in poker..theres not alot of white and black answers..but generally, its harder to fight for the pot against calling stations/loose passives and villians who obviously have something and wont fold..TAGS/LAGS/NITS/TAPS. and better players who will be playing against your own tendencies...
5) you can only really setmine if you have the odds
to do it...so 15-20x relative stack size..need the right implied odds if you do hit a set to make it a profitable play in the long run.