I'm with you, Mac! 'Horses for Courses' - there's no 'right' or 'wrong'!As a guitar teacher I encourage my pupils to adopt the classical position, but it doesn't work too well if you play standing up.
My advice to anyone would be 'try different positions until you find one you like'...
That would also apply to your thumb, too...!!
As for the right hand, my advice is 'pretend you are tightening a wing nut' - the thumb and fingers should pass each other, not collide.
I also recommend holding a satsuma (tangerine!) in the palm of your right hand, and trying to play a little - it puts the fingers and thumb into the correct shape, and shows the correct wrist attitude!
Anything silly helps the memory...