is there really no factor at all if you are using no wind? ... it doesn't really make sense
One needs to consider what the reason is for the factor .. and that is the variability of wind (component). No guarantees along the way that all will be on the side of the pilot .. just reasonable processes and a bit of statistics. If you need/want to have a bit more fat .. then reduce the quoted headwind a bit (or increase the tailwind).
I have never seen a heavy stop to check the TO data when the QNH drops 1 mb. Is there really no factor in a max RTOW?
No factor, per se, but sensible operators will include a process to give an acceptable level of fat to cover reasonable variations. The effect for a couple of mb is pretty insignificant .. and will be within scatter for the calculated gross weight calculations against which you propose to compare the RTOW data. One needs to keep in mind that the starting point for weight calculations (the empy weight) can be a tad off the mark if you are looking to talk in terms of a few kilos ... lots of traps for young players when it comes to weighing aircraft.