"tar-and-chipping" of old concrete flagstone driveway

No u can’t put Tarmac on top of flag stones it will move when you Drive on it

Best option depending on levels and height Use the old flags-skabz as hard core. Smash them up small, go over with a wacker to compact and level. Then you can get your first hit of tarmac on. Will save material, waste and labour costs.

No it will crack

Applying tarmac or concrete laying block paving on laid flagstones i would say no only if underneath the flags a substantial amount of type one or crushed aggregate underneath. If it has soil which in most drives or back gardens have thus will allow movement through time and act as a weak point and and can cause cracks or even sink holes. If your going to pay a lot of money on the top surface I would strongly recommend doing the same on the prep work thorough and proper job