"Now put the pen beteen your thumb pad and your finger pads and try the stretch again. Suddenly an impossible stretch becomes possible. This dexterity grip is what you need to allow you to stretch the needed one-fret-per-finger for guitar playing. That's why you've been told to keep your thumb at the mid-line of the neck.
"

That works as well if instead of the pad of the thumb you use the bit of the thumb where the joint is, and that seems to lend itself to a more natural guitar hold, for me anyway, and easier to exert firm pressure.

To hold the guitar at the right angle a strap is the way. Or the type of neck strap that classical guitarists use probably does it even better, though I've never used one - Willie Nelson does, so it's got street cred!