For some of the various questions:
Each MobileMe account receives five aliases -- these are separate email addresses, but they all go into the main inbox, so aren't really useful for sharing an account. They're good for on-line registration, purchases, etc. You can use different names, and you can delete one or more if you start getting a lot of junk/spam, and simply create a new one. You CAN'T upgrade an alias to anything else.
You can add an "email only" account for $9.99 per year, that person doesn't get any features other than email, but I assume it will be push. They could then sync calendars/contacts wired via iTunes for iPhone like they can now. This account would have separate inbox/password/etc. This CAN be upgraded to a regular account or a "sub-account" on a family pack at any time. You can also delete an email-only account and change the address to a new one. (you CAN'T change address on main account)
A family pack is $149 and has a main account with the same storage and features as individuals, and 4 sub-accounts, each with 5 GB storage (between iDisk, email, photo galleries, iWeb on a Mac, etc) -- they can use all the MobileMe features. Each account has its own email address and password, syncing settings, iDisk, etc.
I assume that the push contacts and push calendars will be similar to the wired ones now. For those that want to share contacts/calendars, you can set up different MobileME accounts to sync (push) on same Mac (PC) account.
For those that DON'T want to share calendars/contacts, just use different user accounts on Mac (and do whatever equivalent is on PC ... ), and different MobileME accounts (a sub-account on family pack gets all the features, just less storage -- but if family pack is shared between two people,the second person could use the extra sub-accounts for iDisk storage, etc)
None of the syncing and pushing has to do with iTunes MUSIC. But two friends with MobileME could put their new music on each of their "public" folders of their iDisk and share that way. (should still put password and tell your friend the password). If it's purchased music, the friend's computer will need to be authorized, as someone said.
(I'm not exactly sure how some of these features will be set up on a PC, I know that you need a Mac for the web hosting -- only works with iWeb -- not sure about everything else)