I no longer have the need to look at the fretboard (I only hold it that way out of habit) so where the guitar faces isn't a problem for me. And I've got the left hand/guitar neck positioning where I'm comfortable. My problem is with the right hand. From what I've read, you want to keep your wrist straight and the line of the forearm purpendicular to the strings. When I sit with the neck horizontal, my arm is directly in line with the strings. If I hold the guitar in classical position, it's at about a 45 degree angle. Where this causes problems for me is in palm muting the bass strings. As I understand it, you're supposed to mute with the side of your hand near below your pinkie. But for me it's the ball of my thumb that rests on the strings unless I really cock my wrist around. And the ball of my thumb is shaped so that it mutes everything, or just the center strings. I can't get positioned to mute the bottom two strings with the ball of my thumb and still be able to reach to play the string with the tip of my thumb.
As for standing up, because of back problems it's out of the question. I can stand in place for a maximum of 3 minutes at a time.