Where did angular momentum come from?

I'm pretty sure convection doesn't happen in an object like the moon that does not have a fluid or plasma core

Of course

If it was fluid once, then that could produce a present day magnetic field, though, yeah?

No, a field is not a physical thing -- it is an abstract concept used to describe the potential for force at all locations. So as soon as the source of the magnetism changes or goes away, the field changes or goes away, too.

Some of the minerals on the surface could have been polarized by the original field, and any molten material which solidified while the field was in place could have been polarized just like magma here on earth.

I think the resulting polarized field would be very weak though in comparison to an active field being generated from fluid movements.

Which would appear to be the case.