There's a possibility that the granite slabs sit on a cement bed (when I dug along the edge of the sidwalk last summer to run sprinkler tubing I encountered what appeared to be cemented gravel on the underside of the walkway). It may be the case that this is there simply to cement in the outside border of belgian blocks. HOWEVER, if the granite slabs are on a cement/concrete base, can I simply pour sand on top and relay the slabs, or would I need to jack-hammer up and remove the concrete?
You can lead a horse to water.......... BUT you........
If the stones are "heaving" it is likely because you don't have proper drainage. Freezing will cause the stones to heave but only if moisture is abundant. If the stones where installed on a proper bed of gravel (to allow for drainage) and then sand to properly "bed" the stones, the problem would be less of an issue.
At any rate - you are wasting your time trying to keep grout in tact if you aren't willing to install the stones properly. And even then it is a gamble. You are dealing with a natural phenomenon and in reality there is little you can do about it.
Now, I'm going to go sit with yes it's concrete and Carlisle and silently enjoy the rest of the show.