IWP and Accounts - not getting the idea

Can anyone recommend a decent schema for the following, because I'm having a tough time of it.

in FM7 (direct and via IWP), I would like to set up a log-in system that allows new users to establish a user account name. Once they have established this account, they can enter the rest of the solution/site and operate with user-level permissions.

Splash screen shows button options for new user or existing user. If new, they enter an account name in a global field, hit the new user button, and the account is created. They would then be automatically relogged-in as that account. If they are an existing user, they hit a button and are prompted to enter an account name and password.

In the FM7 client, this all works fine. However, in IWP, it's a disaster, and certain normal operations seem to be disabled. So I guess I'll pose some general questions because I can't seem to get the big picture on what's happening. I may be brain-damaged.

If I'm in one published file and want to open another published file, how do you do this? Open file is disabled. Linking via a relationship works in FM to automatically open the file, but not in IWP, it seems.

IWP will automatically log a user in as guest if that is enabled. The FM docs (for IWP) say you can then allow the user to change their account using re-login. Fine, in FM7 app. But Re-Login has no dialogue option for IWP. So how do you prompt a re-login under IWP so that the user can put in the account name and password? (Specifying the account and password by reference from global fields might work, but this sort of kills your security model, yes?)

Anyway, I think I'm missing something basic here. Any help would be much appreciated. This should be trivial, no?