Hi, mcrx. DH and I took a course in beekeeping last winter and started two beehives last spring. Yes, you can heat the crystallized honey and it will be fine to use, although it will recrystallize when it cools, so only heat as much as you need. You can put it in a microwave-safe dish and heat it there, or in a water bath.
But honey is not very different from refined white sugar. In fact, all sugars (white, brown, agave, maple syrup, etc.) are chemically very similar and are metabolized the same way by the body. So using honey is still adding sugar to your diet. And box mixes definitely contain sugar.
Btw, I have a couple of chronic health issues so I have spent a lot of time with doctors. I suggest you bring a notebook with you to your appointments to take notes on what the doctor tells you. It can definitely be hard to remember everything later