Infinity J30 transmission problem

I have a J30 1994, AT, 160,000mi. It is a car for my student kid. He was driving in a slow traffic when a sudden loss of power occurred, with gear grinding sound. Now the car starts fine, but would not move in any direction. A transmission repair shop quoted me $1500-2000 to fix it, so I decided to junk the car instead. However, before doing this, I thought it would make some sense to look at the transmission myself, for educational purposes at least. Now, here is my puzzle.

The self-diagnostics shows that there is no problem with any sensor or solenoids, and the repair manual suggests to inspect first the items related to line pressure controls. I found that the ATF was at normal level, and does not look too bad, it contains no visible metal or other particles. I don't have an oil pressure gauge, so I proceeded with "ON-VEHICLE SERVICE". After removing the control valve assembly, I found that the line pressure solenoid valve functions fine. Inside the transmission body, all accumulator pistons move freely and look fine too.

Now, the questions:
1. Is there any way to check internal valves inside the control valve body without disassembling the upper and lower bodies? Something like applying air pressure to certain holes?

2. There are three other solenoid valves: shiftA, shiftB, and overrun. They all have a resistance of 26 Ohm, but when I apply 12V to them, they do not move unlike two other valves. Are they designed to work under some bias pressure, or what?

3. Is it possible that the AT control unit, while it senses all sensors as fine, would fail to deliver actual power control signals, and needs replacement?

4. The manual suggests next to "disassemble AT" and check oil pump assembly, torque converter, and clutch assemblies. From bottom view, all mechanical parts look fine, and no signs of desintgration was found in the pan and retainer. What is the typical failure of those mechanical elements when no signs of desintegration is visible?

I would sincerely appreciate any input,

- Al

P.S. The transmission model is RE4R01A, which turns to be also in 4x4 Nissan pickups, 4x4 Pathfinders, and 300ZX non-turbo cars with VG30DE engines.