6) No blocking was installed where the joists meet the main beam.

My questions:
1) Is it possible to straighten the twisted joists? If so, how?

Yes, you can straighten the joists, but doing so may wall produce
nailpops and cracking wallboard above, because the plywood
subfloor is going to heave a little bit.

You build a big-ass wrench out of dimension lumber,
bolts, and plywood, sort of like a peavy with a fixed
tooth, and pry the joist vertical, and have someone
stuff blocking in there while you hold it.

Where there's something in the way so you can't
put in blocking, bridge across the bottom of the joists
with a 2x4 until you cross two joists that ARE blocked,
on each side.

(see www.goedjn.com/sketch/wrench.gif for the sketch
of the prying tool).

After doing the whole floor, inspect the topside
for things that have popped up.