Or, maybe just normal drawing will do the trick?

..how do I find an angle of an angled wall...

Using the editing tool (arrow), click on either end of a selected wall to display actual wall angle.

..how can I draw a wall perpendicular to an angled wall?

Use text entering - start drawing wall from somewhere on a wall, press enter, add accurate numbers (add 90° to angle displayed above).

..draw a wall from a pair of coordinates to naother pair of coordinates...

Modify wall dialogue lets you set start and end point of a wall, i.e. co-ordinates.

..fairly experienced SketchUp user...

All of the above are generally easier and faster in SH3D than SketchUp. Once you know how. Good luck.
ok