(1) Is there a display of flight control position available to the crew in flight? Both upper and lower rudder?
Yes, there are 2.
The Surface Position Indication on the Status display available on the lower EICAS gives approximate positions of the control surfaces. This display is used to verify control positions when doing control checks.
There is also a maintenance display available on the lower EICAS via the center CDU, though it is not "normally" used. The Hydraulics display gives a higher-fidelity display of control positions than the normal Status display.
(2) Is it possible to (hydraulically) unpower each segment of the rudder? Inflight/air mode?
Normally, no.
There is a set of maintenance switches on the overhead maintenance panel, "for ground maintenance use only." Whether these could be successfully used in flight for this purpose, I don't know off hand, but they supposedly shut off hyd system #1, 2, 3, or 4 (respectively) power to the tail.
To depower the lower rudder, you'd have to shut down hyd systems #2 and 4, which would also reduce the stab trim and elevator authority by half. I don't know if this would be a reasonable alternative to using the upper rudder (plus aileron and engine, when necessary) to compensate.