Does previously unused donated stone in one event (dragon relic) carry over to ROC event?

Yes our 400k stones that we collected during relic were there to use during roc as well

Yes, you can purchase stones from the start and accumulate them for upcoming RoC. I usually collect around 14k to place all my buildings. If your alliance has all 4 alliance buildings that is 16 buildings and if you reset your spec. points to get the extra Guardian/Assualt buildings, the 14k stones should still be enough.