@Craig633
I would forget about the service animals. Yes, it would have been polite for the guest to have informed you but there's not much you can do about it.
I would respond something along the lines of:
"Dear X, thank you for your feedback. We strive to provide a comfortable stay for all our guests and therefore take all complaints seriously.
However, when we last heard from you about the heater, you told us everything seemed to be working fine. We did not receive a response from you when we messaged to check about that a couple of hours later. If there was still a problem, we would have happily sent someone over to fix it as we had suggested to you earlier, but you had told us this wasn't necessary.
The jets on the master bath do work but are turned off for safety reasons. That is why we do not list that item as a jacuzzi, but rather as a regular tub. Had you informed us at the time, we would have happily explained that, but I trust you were able to enjoy the hot tub downstairs.
We sent an inspector to the listing to investigate but he found no evidence of bed bugs or any other pests and issued us with a report confirming this. I am happy to send this to you if you like.
Based on the above, I am afraid you would not be entitled to a refund on this occasion."
OR, "Based on the above, you would not be entitled to a refund. However, as a goodwill gesture, I'd be willing to offer you a partial refund of $X."
If you refuse to refund, the guest may well leave you a bad review, but he's quite likely to do so either way and I personally don't believe hosts should be held to ransom by the possibility of a bad review. If he leaves one, you can easily respond to clarify matters.