It's the clutch... it isn't fully disengaging for whatever reason and the input shaft of the trans is continuing to spin even with the pedal depressed, and anytime there is a shaft in the trans spinning when you select reverse, it grinds.
You already said you are stopped 100%, so that only leaves the clutch, for sure.
Movement, even a little, spins the output shaft.
The clutch spins the input shaft.
Reverse grinds on selection only if a shaft is spinning.... so if the car is 100% stopped then so is the output shaft, and that only leaves the input shaft which is controlled by the clutch so it it the clutch at fault here.
It could be a problem at the clutch parts themselves, it could be something with the hydraulic system, the TO bearing or release arm in the bellhousing, etc... but the grinding is caused by the clutch not disengaging fully and causing the input shaft to spin with the engine.
1st is getting worse because the clutch is getting worse. The 1-5 gears have sync rings, they are small clutches themselves that work in the trans on each gear to match shaft speeds when you select a gear... these clutches are fighting the clutch between the engine and trans (the faulty clutch that is dragging) and eventually they will win and you can select 1st, but it is hard to do so. It is causing excessive wear on the gears and sync rings in the trans, as they shouldn't need to fight the clutch.
The shift cables can cause reverse to grind if they are not allowing it to select fully, but it selects just fine then grinds as you release the clutch... your grind occurs as you select the gear meaning one shaft in the trans is spinning. Reverse has no sync ring, so it cannot match shaft speeds or fight the clutch, and it is also the only non-syncromesh gear in the trans, meaning you actually slide a gear in between other gears. This is why it is straight tooth and whines when you use it, so it can slide back and forth.
All the forward gears are syncromesh, they stay meshed constantly and don't slide. You select them by locking and unlocking them to the shaft they ride on.
If you can select the gears fully and it feels fine with the engine off, then the shift cables aren't the issue.
Oil is important, but won't cause this issue.
You have a clutch issue to track down, but unfortunately I don't have much experience with the 02A/J trans types, so I can't suggest where to look. It might need the hydraulic system bled, it might need a new clutch kit, etc... but it is a clutch-related issue