1. Either by having several save files, or by regularly changing the active household in a menu called 'Edit Town'. However, if you're playing several families in one save, you may want to have a feature called "story progression" turned off since it may change your other families in ways you didn't want to. Also, you could want to turn off aging since those other families would otherwise age. This may cause some problems as well, since then no other families in the neighbourhood will age or get a job, a child, a new house, move etc.
My personal preference is to try to keep one save per family, but that's just me.
2.In the normal neighbourhood, no. One lot per famliy at any given time (you may of course move). However, you can buy several vacation homes if you have WA, but you can only designate one of them as your current home lot when you're in the actual vacation (you may of course change this as well, so in practice it's just a minor note).
3. Sorry, someone else may have more info. I have only downloaded Riverview. Ever.
4. You can buy at the stores with usual simbucks, but there is a 'special merchant' that only takes "ancient coins" which you can find in tombs. These are not always necessary, but can be used to buy a nice reward.
EDIT: Pam you made it before me, but I have some other info above.