Hi guys, recently started learning how to play the intro of Stairway to Heaven.

I can get the barre chording down, but the issue is that my thumb is never able to go above the fretboard (my teacher used to say I should make this a habit) and I when I do barre it, my finger tends to go at an angle rather than perpendicular. The same goes when I use my pinky to play notes further away. I tend to be almost unable to reach and sometimes I get a "weak" sound.

Is this okay? Do most people do this? I really do not want to use the "small hands" excuse here but I just wanna know if this an acceptable method of playing, many thanks!
I never stretch my thumb over the fretboard. I keep it firmly planted in the center of the back of the neck, and I have no intention of changing it. Some chord shapes might be easier with the "baseball bat" posture, but I would never do full barre chords with it.

And do you mean that you don't bar the fret with the underside of your finger, but instead your finger is slightly tilted so that more of the side makes contact with the string? Because if you do that should be fine imo. Might want to check that though.

And well, you just need to practice the pinky. And you should, it's really important.
I had to go grab my acoustic and see what I do...I never think about this stuff...

I use my thumb on the back of the neck to support my fingers for the barre A section then wrap it around the neck (to muffle the top E string) when it goes down to the D then F. I tried wrapping my thumb for the barre A, didn't work too well, I can't keep enough fretting pressure to get clean chords.

Basically whatever is comfortable for you, there's no strict rule. Other people may not do things the same way I do, I don't worry about it. There's that no strict rule thing, and I also have a damaged tendon in my left ring finger that causes me no end of grief and had to modify the way I play a lot of things many years ago to play at all.
Hmmm...I wonder what this button does...