I thought I'd check in to post update, in case anyone is interested and to solicit further comment/advice on how I'm doing.
I have completely rebuilt the Reverser Control Valve (called RCV henceforth), and reinstalled it - clean as a whistle, all valves/spools move freely, all lands smooth, etc - I still have the same problem: everything works fine, except the tractor will not move forward. Now I'm looking to the clutch/reverser housing, but want to make absolutely sure before I break the tractor in half again, so I checked a bunch of things (all measurements with tractor warm @ about 2,000 RPM):
1. Rechecked all pressures: 155psi at RCV accummulator; 0-155psi at reverse tap (clutch depressed to engaged); 0-0psi at forward tap (not 10, not 5, not 2, but 000).
2. Connected tubing to each port to verify flow: get really good strong flow from reverse tap as clutch is engaged; get just a trickle from forward tap as clutch is engaged.
OK, I understand this is still not conclusive - if fluid is being dumped back to sump inside the housing, fluid will still follow path of least resistance, not building pressure/flow at the tap.
I am inclined to try one more experiment before I give up and put tractor back on jacks, and am curious what anyone might think about this: I have fashioned a plate from 1/8" thick aluminum and have cut all holes in it so it looks exactly like the gasket between RCV and housing, except that the holes that go to the rev & fwd oil lines are just tiny pinholes (guaranteed to create back pressure and identical to do comparative testing) [should be OK since lube lines are separate, will still limit to short durations for safety]. I will again be looking for same pressure at fwd & rev taps. If pressures are significantly different, then has to be problem in RCV (although wouldn't preclude multiple problems). If pressures are about the same, then RCV has to be OK and problem is definitely in housing.
Only other thing I've considered, remembering from when I rebuilt automatic transmissions: In some freshly reman auto trans, there can be e pressure spike at first run that can crack the aluminum pistons in the clutch packs (happenned to me once, had to tear down again and replace w/ steel piston). I think the pistons in the reverse brake and fwd clutch pack are aluminum. Is anything like this ever known to happen in tractor hydraulic clutches?
Any thoughts or suggestions???