1.Why do many gliders have two variometers?

Sometimes its for redundancy (one comes with audio, the strictly
mechanical one doesn't), sometimes one might measure total energy
(and thus compensate for stick-lift) and the other might not.

2.Assuming for a moment that you are not flying in the flatlands,
how often and under what scenarios do you use your compass? What are
some good exercises for developing airborne compass skills?

Simply speaking, when orientating. Wether that is orientating to
guess on what slope the sun must be perpendicular or orientating
where I should go.

3.If you needed panel space for a PowerFlarm display, which
existing instrument would you delete (if any)? (Assume a PDA is
present.)

If it occupies panel space, remove the FLARM.
If not, remove any instruments which are not strictly bound to the
panel (PDAs, sometimes accelerometers) and can safely be
repositioned (do not block view towards important handles for
example).

I've my own answers already of course, but I'd like to have the
benefit of your experience/insights.

Most of my experience is in aerobatic gliding, but I've thought
about my own (B4) panel a lot.