Most flight schools do not have renters covered under their general liability or business insurance policies. Technically, for your own interest as well as the flight school, you are suppose to maintain your own renters insurance for aircraft you use and do not own. It is possible for an FBO or flight school to have renters insurance covered under their policy, but most do not have that or even know it's an opportunity. If a flight school requires you to have it, and you do not, then the insurance company has every right to subrogate on you for something that is your fault.
Just because you sign something with the flight school negating their right to subrogate on you means nothing. It's not up to the flight school. That has to be contractually bound with the insurance company writing the insurance program for that flight school specifically naming you.
That price for $40k in hull is not to far off from what is competitive in the industry. They are requiring that much in hull non owned because that is probably what the aircraft is worth in full value. Just because you have non- owned insurance for the deductible does not mean they can't subrogate for the full amount when you are responsible for a total loss or damage that cost more than you carry insurance for. The insurance company will be reasonable and most likely won't come after you for the full amount in most situations, but it's always good to be on the safe side when it comes down to protecting your own assets.
With respects to the number of seats question, there typically will be a difference in liability cost for 4 seats compared to 2 because there is potential for a much higher loss if you have 3 passengers with you compared to just 1. In a fatal crash, that is 4 total lives they are responsible for, not just 2.
The limit of $100k or $250k are just a few liability options to chose from. You can pay more for $1mil smooth in liability, but that's up to you if you consider it worth the additional cost for the coverage. The limit reduces the insurance companies exposure and that limit is PER PERSON. So if their is a total loss and causes fatalities or injury, each person could get up to $100k (or what ever that limit is) and the remainder of the million could cover property damage.
I am new to this forum and am a student pilot out of Dallas but have been in the aviation insurance industry now for about a year and a half. I am close to soloing and will be needing to get non owned coverage as well. I work with Falcon Insurance and would be glad to help you with this.