Agreed on the seals though I didn't mention them as I consider seals a 'normal' replacement item when I open up a high mileage box. If those bearings were going, I would think you would have fore and aft play in the shaft and shifting in all gears would be compromised if the play were large enough. Or, you would have play fore OR aft and then depending on which way, you would have problems with 1,3 & 5 or 2 & 4. I don't have a lot of experience with the 5MT, but I can tell you that in all the rebuilds of other manuals I've done, whether they were AWD, FWD, RWD or 4WD, individual gear grinding is always (almost...) a blocking ring itself or in combo with a synchro. I'm not discounting what you are saying, I'm just thinking out loud. Also, with fore or aft play, you usually have a rattle due to the shaft walking back and forth. It's easy to check once it's open though. Based on what you've see, what I've seen and what he's got, he's not going to know until he tears it down and looks. The bearings will be easy to check and obvious if bad. The blocking rings should have little grooves inside them running the inner circumference of the ring. You'll have to compare all of the blocking rings to see if the grooves are the same depth. Ones that are shallower are worn and not applying enough pressure to the beveled face of the synchro to slow the gear down enough to allow the synchro to mesh and shift properly.
On the note of I75ea.... Sounds like the input shaft or front main shaft bearing is going bad. Put it in 4th or 5th, engine running with the clutch in and see it the noise goes away. I believe (not 100%) that 4th gear is a 1:1 ratio and locks the input shaft and main shaft together. It is/does with every other trans I've worked on. If the noise is not present in 4th, engine running and not moving but is doing the same with any other gear, it's the input shaft bearing. Of course, now thinking back I believe I'm wrong in that it's 4th gear specific for these trannys. Any gear will lock the input shaft and main shaft together because the aren't in line (front to back) with each other. The input shaft is on top, and the main shaft (or pinion shaft in the case of Subies) is under it. In this case, if it doesn't rattle in gear, clutch in and does rattle out of gear clutch in or out, then it would be the input shaft bearings Gloyale mentioned. Oy... My brainpan is starting to hurt. lol