Is our leftover concrete, not poured from the truck and returned to the batch plant, counted as diverted from landfills per MR2 if we can provide documentation that it is reused by the batch plant in some way?

I see in FAQ's:

Can materials that have been unused and returned to the manufacturer as part of a “take-back” program contribute to the credit?

Yes, materials that would otherwise be waste, but that are diverted from the landfill to be salvaged or reused can contribute to MRc2.