So if I weigh 60 kg and my 3 fellow bench sitters weigh on average 110 kg, what is my protection level?
Impossible to say, you would have to run your own series of tests, and I would assume it would depend on which particular combination of seating the four of you were in ie you on the outer, or between two of your comrades.
What is the maximum allowable seat loading
I don't see that a max loading is a requirement of the FAR's. The only time I've seen a max load stipulated was for an EMS stretcher, and the general opinion of pilots was that was down to the crane wire strength used to lift the stretcher to the door for loading.