I can take a few of the rules questions you asked.
First, I want to congratulate you on your excellent accomplish and commend you for the excellent manner in which your tale was told! Great job!
Now, back to the questions. Keep in mind that Ironman is an informal variant developed by several different groups of people at different times. The rules hosted at DSG/RBD are an attempt at consolidation but you will see people who play by slightly different rules. This is even more true for the NM/Hell continuation rules, which were mostly thrown together by the IM winners who have participated in this forum.
One player has to be alive when Diablo dies in order to get credit for an IM win. So even though it is possible for Diablo to die via firewall with all players dead, it is not allowed. However, once Diablo is dead all slain characters get a free res. As KoP mentioned, if you want to continue to NM difficulty you should attempt to recreate L16 and finish the monsters you had left behind first.
IMO you cannot "save" shrines to use on a different difficulty level. This seems like a rather silly notion... imagine your characters trying to carry the shrine with them However, at the start of your nightmare game you DO have access to town again. If you guys decide to continue to nightmare, you should look at the rules regarding this (hosted on RBD site in the D1 variants list).
Regarding the strategy concept you mentioned: Yes, there are items that cannot drop on normal difficulty. However, IMHO the overall quality of drops does not improve significantly enough from one difficulty to another to worry about it. Besides, if you are already planning for nightmare difficulty at the start of a normal IM game, we need to start making the rules harder This isn't supposed to be a casual win.
Posted on Apr 7, 2001, 11:02 AM from IP address 22.214.171.124