In California and most of the United States, most occupancies need at least 2 types of approvals: planning (also called zoning) and building.
The planning code will cover issues like the amount of parking required, the permit to have an assembly use in a neighborhood that might not normally have such uses, etc. There may also be related issues such as how cars circulate in and out of the parking, their impact on neighborhood quality of life, etc.
These planning questions are usually very specific to the local neighborhood and community, and typically cannot be answered on a site dedicated to building codes.
The building code will address issues related to safety and health: structural design, building materials, fire safety, emergency exiting, accessibility for persons with disabilities, sound isolation between the A assembly occupancy and the R dwelling units, heating, cooling and ventilation, seating arrangements and maximum occupancy of the room, related restroom facilities, etc.
Please also note that there are fire codes which are similar to building codes, but they also deal with firefighter and fire truck circulation around the building and site, so the fire code might affect your parking driveways.
Let's look at your questions:
Local planning allows us to have very large residential building (rural CA).
We plan to have assembly space of about 150 persons (15*150~2300sf) within the building.
Given fire barriers, fire sprinkler and 2 exits provided, what else can typically be of concern ?Y (Yes, we can accommodate parking).
BUILDING CODE QUESTIONS?
If AHJ approved, say 170 occupancy load, will they need to obtain specific permit for every gathering event (private religious cult) under 170 OL ?
PLANNING DEPARTMENT QUESTION.
I do remember in multi family building, you need special permit for rooftop meeting room that has assembly gathering over some OL.
Thanks in advance.
PLANNING DEPARTMENT QUESTION.
You have raised many complex building code issues that cannot be resolved with simple answers. For example, an assembly occupancy might not be allowed on an upper floor in a building made with a combustible wood frame; or it might be allowed if a certain kind of sprinkler system is installed.
In my opinion, you should first check with your planning department, and once you have determined that they will allow the assembly use, then you can consider how to design to meet the building code.