I have 125 kohler diverters in my hotel (kt-15101-4). Half of our hotel is withought hot water. I have shut off all public water supply for that end of the hotel. The only thing left open are the guest showers. Is it possible for cold water to get into the hot water line even with the shower off?

Yes, that possibility exists. Question: did you have any (any) plumbing work done on your property before this problem happened ? Like replacing cartridges etc. Back to you. Milo