I dont have experience with L's that old, but Ill take a whack at it anyways.
1. I dont know
2. On newer citizens (2001 model) there is only 1 set of parameters, not independent main/sub parameters like on a Fanuc. There is just "Input" or "output" parameters, no option for main/sub. Are you sure there is in fact an independent "Sub Parameters"?
3. To modify the PLC data, (on our machines) you have to go to MAINT -> I/F DIAG, then type "1001" (first box on the left) and "M" into mode (the 3rd box from the left). Press input. Nothing will happen, but you can now change the PLC data.
(4.) Ask your local distributor. You're most likely to get reliable information from a maintenance tech who's been around a while.
Good Luck!