I'm a couple games in so I can't guarantee this is 100% correct, but here's my take:
1. On initial setup, how many buildings are placed out in the building slots? Just at the greyed out location mats, player mats don't receive a building card, correct?
All locations except Town Hall should receive a building in their buy slot (show with the buy phase symbol). Greyed out mats and the sheriff's office should receive buildings in their porch slots.
2. Then, in subsequent rounds, all building slots are filled including player mats and greyed out mats, correct?
3. The greyed out mat buildings come from their porch slot and a new building is drawn to renew the porch slot, correct?
If the buy slot of a greyed out mat or the sheriff's office are empty, the building from the porch slot is moved to the buy slot and a new building is drawn to fill the porch slot.
The buy slot for a player's mat should be refilled directly if it is empty.