Scenario 1:
That seems like an unlikely scenario. In most cases on scripting channels I've been a part of, if a user keeps leaving then 5 mins later comes back with another question he gets the standard "If you have a bunch of questions, just ask them, don't keep join/parting" Additionally this will lead to things like, "SomeOp was helping me before, but then I parted and now he is gone, where did he go?" whereas if the user had been in the channel he would have seen the "Quit: brb" message and not had to bother anyone with such a question.

Scenario 2:
If the user can't even remember the channel name, then they can look in the status window for the /names reply. The whole part about remembering the key is also irrelevant. If the user can't remember the key, how does the said user intend to rejoin the channel tomorrow (after mIRC/PC have been shutdown)? Channels I frequent that require a key, I either remember the key, or I write it down somewhere. And, if that's not good enough for you, the user can go to the channels folder, click "add" and enter the channel name and password. This, is a better solution since now even after mIRC is restarted the channel name/password are still stored. Additionally, if there were 1200 nicks talking at once, upon rejoining the user will most likely /clear anyway in order to eliminate their full buffer filled with text they don't even care about.

Scenario 3:
Again, the /names output is displayed in the status window. The oper can join, then part and see the names reply in the status window and do whatever he/she must do there. Additionally, most networks allow the opers to simply do a /who #channel or /names #channel from outside the channel and see all users (including +i) therefore eliminating the need to ever join the channel if you already know the channel is bad.

I really don't see how any of the scenarios you provided are in anyway real life experiences, and I really don't see how the idea of keeping the channel window open on parting would make life any easier even if these scenarios did occur.