"The first has to do with the direction of reading. I believe the second is, like many things in typography, customary; there’s nothing inherently better than right-leaning letters, except we’re used to them since those are the only ones we ever see."

Not at all. It is simply because the tip of a quill and the nib of a traditional[1] fountain pen cannot move in any direction. The way you hold it if you are right handed forbids moving in the "backslash"* direction[2] upwards. You can "draw" from top down but not "shove" upwards in that direction. Since in joined script we start letters mostly from the baseline (and have connecting strokes) this makes the "right leaning slant" much easier to write.

The anatomy of a right handed writer also plays a role. When you write fast, the hand pivots at the wrist and fingers as it moves rightward, and that produces a forward slant.

[1] Modern fountain pen nibs have a little ball at tip. So even if you used on in primary school you might not have noticed the directional effect.

[2] That is why the slash (virgula) has existed for centuries while the backslash is an invention of the computer age.