I wonder if weight and balance is the ..
Probably not. W&B is readily addressed by routine pilot stuff, albeit that the calculated numbers are a tad rubbery.
CAAP 235-1(1), p11 is relevant regarding standard weights in small aircraft. The CAAP is based on a detailed report by John Klingberg, then an airworthiness performance engineer with the Authority (and a nice bloke to boot). If you are able to track down a copy it makes for excellent relevant reading.
.. passenger seats in a C152 is zero
Perhaps one might first read CAR(1988) 2 "crew member" which includes "crew" before offering conjectural comment. Likewise for C172. The sensible interpretation is that we are referring to minimum crew rather than available crew positions/stations. On that basis, one's C152 can have 0/1 pax seats and the C172 2/3.
CAAP 235-2(2) is essential reading for the carriage of small children in aircraft. The CAAP is largely based on earlier AWBs.