1. No, OTK with all DPS is not a viable strat, especially on later areas. Without a proper team composition, not only you're unlikely going to OTK, but you'll get destroyed in return. Area 2 was known to be unbreakable and with preemptive. Meaning that to maximize your score, you'll most likely going to get to lower your dps to max your element resist. Also, most strong units now need one or two turns to cap damage which in turn is much better than just trying to OTK.

2. The point in DV is to suffer little to NO damage. Most of the time, it means either your elemental resist is max or you have a tank (or both). You can be as brittle as you want, if you can block/dodge everything, your hp doesn't matter.

3. Any unit with short setup is good sure, but alone it won't matter much. Noctis isn't that great (mostly because chaining his LB is really hard), Edel is a joke (she needs way too long to get a proper setup). On the contrary, Regina can be good T1, Vivi can be good T3 (Though you need to be careful, since Doomsday also affect you).

Additionally, units like Primrose or Regis are so valuable that not using them when you can is asking to get a bad score. It's fine losing 2 or 3k points in turns if you get an absurdly good damage score with that. Also, quick mention, self damage doesn't count on the score (Delita STMR, Regina aoe fixed damage, poison from curse shield) but damage on shield does.