... if you happen to be requesting hashes where you've received a couple dozen reduced hashes and then the network has a huge spike in difficulty. This is a potential liability issue so far as the "owner" of the hash farm server is concerned, as it would substantially impact the "profit margin"
Sorry, you misunderstand. The server operator only pays out to easy hashes that were submitted for a block that is won. There's no payout for easy hashes that were submitted for a block that someone else generates.
The difficulty can't change in the middle of a block. It always changes at a known time, and you always know exactly what the difficulty is, so there's no risk here.
...you are dealing with probability here and not mathematical certainty...
That's absolutely true, but over time probability tends towards mathematical certainty. If the server operator is getting a few percent of the generated bitcoins, it doesn't take long to become virtually guaranteed profit.