the seating policies should be made to bend, given different situations. hostess should either keep floor manager close, or be able to make decisions that are proper and prudent. if a single person walks in during a rush and exclaims that they have a party of 20, space should be set up for that party, period. if you have a ton of people waiting at the door for a seat, and they see all those available(for a party that hasnt arrived) then you have a problem. first, i would not have reserved a spot close to the front door. second, i would have had the server put water pitchers and anything else available on the table to make it look more like it was being in use, not just reserved. third, i would have made the singular person exclaiming of a twenty top party wait for ten minutes to insure room for other patrons( rule of thumb.....40 seats avail when seating 20).
another way is to have available reserved signs on table.
but i would never, never exclaim the party had to be present! even if that is the policy, i would find other ways to stall, or break the policy.
as far as seating parties close to each other in an otherwise empty dining room, one must remember, servers are generally given sections of about a half dozen tables. during slow times, the rest may cut down to one server who is still using the same 6 tables.
the host should be more aware of proximity atmosphere complications and seat accordingly.