My question is, is it okay to take them one at a time? So go up with one, land and swap pax before going again? This seems a little dodgy, as it doesn't really fit with the "would this flight happen without the passenger?" question. Is it okay to do this?
I just wanted to check I'm legal before commiting to anything.
Yes, you are.
Many private flights wouldn't take place without the passengers!
If you are renting the aircraft for the flights, the passenger(s) can contribute anything up to, but not including, the full rental cost. Technically you have to 'share' the cost, so you must make, at least, a nominal contribution.
I can see Whopity's point about the rental incorporating a proportion of annual costs, but;
‘Direct cost’ means the cost directly incurred in relation to a flight, e.g. fuel, airfield charges, rental fee for an aircraft.
specifically includes '...rental fee for an aircraft' in the definition of the term 'direct cost', for the purpose of this rule.
As to your reluctance to 'push it too much' towards what you see as the edge of the aircraft's takeoff performance capabilities, I can only commend your line of thought in the highest possible terms!
MJ
Ps. As a matter of interest, what model of PA28 are youi using?