For whatever it's worth, here is an opinion from another building official:
You did not indicate what occupant load you are considering that is not
realistic. But I would start by considering mercantile (store) occupancy
which gives you 30 square feet per occupant for the entire area.
Another approach may be to use garage occupancy (200 sq. ft/occ.) for the
areas where autos are located and use assembly (exhibit room, 15 sq.
ft/occ.)occupancy for the areas where people are going to be gathering.
If you ask the building official and he/she is willing to contact ICBO, there
will be no charge for the interpretation.
Ben Yousefi, SE
San Jose, CA