Don't use the word 'playable' or 'unplayable' like that. The situation has a big effect on which hands you should play -- opening UTG is many times tighter than defending your BB against a SB open, for example. Only a small % of hands are objectively unplayable as in you wouldn't enter the pot in the scenario that calls for the widest ranges.
The double-suited JT65s are okay hands, even rainbow JT65 is bad but still can be VPIP'd in certain wide range spots. 5432r utterly blows. Borderline unplayable.
Gaps at the top making rundowns unplayable?
QT98ds is still a great hand. All else being equal, gaps at the top are slightly worse than gaps at the bottom. That's the sober way to put it.
Some of the ultra-low rainbow single pairs are just about bad enough to be considered unplayable. K722r, lol. 22xx-55xx with literally nothing else kind of suck. It's situational whether AA72r is a good hand or a pretty good hand; would never describe it as 'unplayable'.
Other questions: JT98 is way better than QJ65 because it flops straights more often, straight draws more often, and the straight draws it does flop are better on average. That's really important stuff. Blocking own 'outs'? Well you have
a lot more to begin with, so to speak!
Low suits are better than nothing. Low suits are less important than high suits -- AdTd3h2h is better than AdTd3h2c by a smaller margin than Ad3dTh2h is better than Ad3dTh2c. But even the lowest flush is still a flush where you'd otherwise have nothing, winning a higher % of pots and thus getting more EV (as long as you don't play badly lol). The importance of non-nut suits is proportional to the width of ranges. Q-high suits are pretty good heads up against a wide range, for example, and their value drops in multiway pots and where tight ranges.