Buying from Bank

If you've already paid all the purchase price and received a deed -- actually gone through a settlement --then you'd better get a lawyer to go after those keys for you.  I don't see how you could move in without them.  But if you're already the owner, you're entitled to do just that.
If you've just been approved but haven't gone to settlement yet (received a check for your mortgage loan, had a closing etc) then all I can do is advise patience.  It can be frustrating buying bank-owned property.  TAlk with your present landlord about staying on month-to-month.  I don't see them throwing you out with a baby, and if it came to an eviction I suspect a judge would be sympathetic to you.