not sure if that heat differential is achievable on the same heatmat but Tim and TeeGee may elucidate.
I have a cable rather than a mat but it is the same principle so far as I know and no! you can't vary the heat as such.
Having said that I have had to do a bit of experimenting this year because my themostatic propagator broke down (element I think) so I propagated everything on the hot bed. Like you Barnowl I liked to use my propagator for high temperatures and the hot bed for lower temperatures.
This is how I conducted my experiment;
High heat; Tray directly on to the hot bed with a propagator cover over it.
Fairly high heat; as above without cover.
Lower heat; I sat the propagating tray on top of an upside down standard tray meaning it was about an inch and a half above the hot bed. I also used a propagator lid or left it off as required.
As you can see in the picture each tray contained 4 sub trays (punnets) and on occasions these were sub divided meaning on occasions I had 12-16 varieties on the go in one tray.
This is feasible when you have a rough idea of germination times i.e. you can propagate plants with similar germination times.
I must say; I was quite happy with the results so I won't be forking out around £70 for a new propagator.