Correct point, IMO, is thumb's last segment or tip segment. Some people force pressure at tip joint from thumb pad muscle, which hinders other hand muscles. Use more thumb tip, muscles come from arm, but as said, you should actually avoid thumb pressure.
On thumb position behind neck, it should move toward its natural closing as a fist, without lateral tension to move behind some finger. The other 4 fingers that should spread out sometimes to reach wider positions like first positions 4 frets, or stretches. So sometimes thumb is behind finger 1 or 2, depends on your hand size and guitar region, but it is not thumb moving to the side behind some finger, it is finger 1 and 2 spreading out toward thumb's left.
About straight wrist, I think it is a modern misconception. Certainly straight wrist results freer muscles movements inside wrist. But I think internal muscles relaxation, dissociation control to tense only necessary muscles, and economy of movement is by far more important. Wrist bend is necessary a lot of times to reach difficult positions, spreading fingers etc. I think using wrist is easier and faster than moving elbow too much far away from body as some players do. You can watch several great players videos and check that. Specially if you are small and have small hands, wrist bend will help with straighter finger and easier reaching. Check Cecilia Siqueira in this video, she is very small and has small hands and use o lot of wrist bend, with no difficulties or health problems:
Her partner Fernando Lima uses less bend, but he is taller, he has big hands and fingers.
Check this Carlevaro lesson, on reaching positions with wrist bend rather than elbow moving out:
I prefer his school of LH positioning.