Hi Heather,
I had a period in my early 20's where my cholesterol was high at 120. As a child, and up until my early 20's, I basically ate whatever I wanted. I was allowed to eat any high-fat, high-calorie foods that my little heart desired, because I often had poor weight gain. But ever since my late 20's, I have had a problem with gaining too much weight. My cholesterol is fine now, but my weight is definitely too high. I just got up to my highest ever at 190 lbs and I'm almost up to what I was when I was full-term pregnant. I really think it is "catching up" to me.
I wouldn't worry about the high-sugar tube feeds though, because CFRD will happen regardless of what types of food we eat. That is because it's like having Type 1 diabetes, which is different than Type 2 (which can be prevented by diet).
But what you're concerned about with Alyssa is what I had questioned my whole life. I always thought that this high-fat, high-calorie diet is not probably the best solution for us in the long-term. I think it is important to just keep a balanced diet and feed her what you think is right. There is no research out there against our classic CF diet at this point.