I was taught lower case x starts at top left, does the arc to bottom left, then goes to top right, arc to bottom right, all in one stroke.

The upper case X didn't have a horizontal line in my case, otherwise it's all pretty much the same as this 1941 doc.

This is useful in Maths, where that letter can be confused with the multiplication sign.

Probably not, at least in my case it is just some lower left to top right line, then the crossing line starting from the top left