The brake/clutch reservoir was about 2mm below the mesh filter level, so topped it off (3/4 of the way up the mesh filter) with more DOT4. As a side note to anyone else following this thread, the folks out at ipd noted that the slight drop in fluid level was due to brake pad wear. As the pads wear down, fluid is drawn in to displace the space in the calipers, thus running through a set of brakes might displace about 4~5mm (vertical) of fluid in the reservoir.
Discussing the shifting problem with the folks at the dealership following a test drive this morning sussed out that a bent clutch/transmission fork in the tranny might prove to be the problem.
Volvo dealer's assessment (brainstorming w/ mechanic & head of shop):
There's no mushyness or lack of rebound to the clutch pedal (clutch master cylinder).
There's no grinding sounds during shifting (throwout bearing / slave cylinder).
There's no clutch slippage under load (clutch disc / pressure plate).
The car has +90K on it and the problem began abruptly about 10K ago, ruling out a bound transmission input shaft.
The problem is sporadic, operating fine one moment and near impossible to shift the next (post pothole, road seam/gap, etc.), pretty much ruling out cable linkages (but would still be worth checking in the dealer's opinion).
And thus, by process of elimination, the Volvo service department figures the problem to most likely be a bent (clutch) fork in the transmission. $$$$