Legacy moveset and mediocre IV pokemons... Worth power up?

No to #1 since it's too expensive an investment for a meh IV. Yes to #2 max it out. Only legacy moves w/ meh IVs worth considering brining to level 30 or maxing are DB Nites, IS/IB Lapras, C/SB or maybe ZH/SB Eggy, and BS Snorlax. Anything else including Mud shot Golem meh.

Z/B: Recheck for exact IVs. If it has high Atk IV, then it could still get a pretty good CP. Even still, if you do not have another Z/B Snorlax it might still be worth it.

L/B: Only if you like using this as an attacker.

That Zh/Bs might tempt me, but I am not in need of defenders...if I was and I had candy then this would be at the top of the pile.
It is a nice moveset, and the IV distribution would factor into the decision.

Personal choice comes into this one, as much as any rational thinking.

I think u should just leave them as they are. 1st one got the best def movesets but too low in level and IV, 2nd one has a good offensive moveset and already reach a decent level (~30?) with high CP that can be used either offensively (due to the moveset) or defensively (due to the high CP). I typically dont power up PM over lv 30 cause its extremely inefficient in stardust & CP increment.

ZH/BS is a great defender, even with these IVs, if you have the candies and dust, I'd go for it.
The other one is good as is.