Is the Cactus Juice Resin reusable? That is, after stabilizing some blanks, can the resin not absorbed into the wood be saved to be used again for another batch of blanks?

Yes, the unabsorbed resin is certainly re-usable.  Some woods may discolor the resin but that would be up to the operator whether or not to use it.  You can always add dye to any discolored resin and it will mask the discoloration.