Your one part looks a little confusing.
Please break it down?
Basically the system works like this (and should continue to under mobile me)
Single User = 1 email 20g
Family Users = 5 emails 5g ea = 20g (yes it's evenly spread)
Single User + 4 extra emails $100 + $40 (rounded up) = $140 a year
Family Users = $150
Even buying extra emails they are considered 'sub accounts' and still fall under your "MASTER" account. They are not alias as each of them will have their own seperate 'inbox' etc. However, the concept is similar if you buy a service from a web hosting provider. Let's say with webhost4life your allocated 200g of email storage and allows 500 max individual email accounts. That doesn't mean you get 500 x 200g = (a whole shatload of disk space) it means 200g is evenly distributed (or allocated in admin so some have more some have less) between all 500 accounts. It's not a new concept and Apple is doing nothing different here.
However, if you only need 1 extra email, just BUY ONE extra email. 20g will then become 10g each OR YOU CAN configure how much each email can have for mail storage.
iDisk should still remain under the master account holder but you can allow others to view, upload, share etc.