When Olive is too heavy for the sidecar, the rear motorcycle wheel lifts up. The combination would not have tipped up as Olive was sitting over the side wheel and as there are only three wheels in total, there is no other way in which it could have tipped.
At the start of the film, Stan and Jack are ticked off by Blakey for being late leaving the terminus. Almost as soon as Stan walks off-camera, Jack rings the bell and the bus moves off. Stan could not possibly have got back into the cab in time to drive the bus away.