Hi Tarrega Fan,
I agree with moth that has been said here, some very good advice. Let me give you another perspective, which most are tired of me championing on this forum.
1) Yes, the wrist is generally straight, as Kevin G. shows in his excellent video. But as Frank N. says, it should be "gently flexible " when it needs to do so.
3) Yes, the fingers deliver more force when the tips are perpendicular to the strings. It is generally not possible to do this all the time, though, but extreme angles to the string should be avoided.
2) This is where I must disagree with most teachers. My apologies to everyone I'm about to offend. For beginners, the thumb should be behind or to the left of the first finger (or behind the first fret). This thumb position is a companion to the 4th finger approach (which is the practice of using the 4th finger on the 3rd fret of the treble strings, again, for beginners.)
In the video at 9:46, when Kevin is demonstrating poor technique, you can just see the thumb has moved into the palm, behind the second finger/fret. When Kevin demonstrates proper technique at 11:56, you can see that the thumb is behind the first fret. Kevin, like Sor, is teaching the 4th finger approach.
Here is a more detailed explanation from my site.
http://larrymcdonaldguitar.com/instruct ... sample.pdf
After using it for several years, I came to believe that the pencil demonstration is not a valid analogy since you need to have at least two fingers and the thumb to make it work, 3 points of force shaped in a triangle. In guitar playing, one of those points is the counter-force from the right arm, which is way, way to the right. So, only one finger needs to be "on the top pencil", and the thumb can assume a more natural position to the left, supplying much less force.
If you use the "'older' third finger approach", then the thumb needs to move into the palm.
All the best,
[editied for spelling, twice!]
Dr. Lawrence A. McDonald, D.M.A., Art Kaplan Fellow
Author of The Conservatory Tutor for Guitar
2018 Michael Thames "Ancient Dragon" Cd/Ir
2008 Michael Thames Cd/Br
Royal Conservatory Advanced Guitar and Theory Instructor