For somebody:
1 Who buys the land (if it wasn't an outright gift to the society, from a r&f member)
Generally a Corporation made up of local Witnesses, in some cases it's a board of trustees.
2 Who borrows money from who for mortgages on new Kingdom Halls?
The congregation(s) borrow the money to build KHs from the Organization, usually. For Assembly Halls the money is supposed to be donated by the Circuits in the catchment area before they start. Sometimes the Organization makes some sort of bridging loan but they really want the things cashed out when they are built.
3 Who physically builds the halls?
Volunteers, the Regional Buildin Committee.
4 Who pays for maintainence of the building?
Maintenance is paid by the Operating Committee which may be the Officers of the Corporation or the trustees. That varies a lot depending on how many congregations are involved, etc. Technically it's the responsiblity of the Corporation or Trustees.
5 Once the building is paid for, who owns it?
The Corporation or Board of Trustees, as representatives of the Congregation(s) The Corporation or Trustees could actually sell the property out from under the Congregation but the only time I've heard of anything like that happening was in Bonham, Texas.
6 Who's name is on the deed to all the land and halls?
The Corporation or Board of Trustees, as representatives of the Congregation(s)