2. Contact the lender and ask.

3. The proper term is "assumption with qualification" in otherwords, you have to qualify to assume the mortgage just like you would if you got a new mortgage.
The only difference in the closing is that you might not have come up with a down payment.

4. You can pay the remaining in cash. You can get the seller to hold the entire $50k. You can get the seller to hold and 2nd and a 3rd, and sell the 2nd to a private investor. Or you can get the seller to refinance to 90% and you take over the payments or assume the new mortgage.

5. There can only be one 1st on the house at a time.

6. Yes that's possible. You could also take over the "non-assumable" 1st sub2 and then have the seller hold a 2nd, or you can give them a down payment, or you can borrow money for the down payment.

7. I don't see why not.