Muffins are fluffy, but crumbly
It looks like you substituted fully a third of the total flour in the recipe with coffee flour. Admittedly I have never used (or even heard of) coffee flour before, but I bet it does not come out as sticky as regular all-purpose flour when mixed with water (or milk, coffee, etc.)

You can make paste (for sticking up posters and wallpaper) with wheat flour and water, so if your muffins came out crumbly and wouldn't stick together, I would suspect the coffee flour, or rather a lack of wheat flour. That novel ingredient does sound like an interesting idea, but one third of a critical component like flour might be too radical.

Both the milk and the coffee in the recipe are primarily water (at least for structural purposes), so I doubt if that substitution was your problem. As far as the butter, one stick sounds like a reasonable amount to me, and lack of fat doesn't usually cause crumbliness.

It would be a good experiment to make the muffins as specified once, just to see what they are supposed to be like, and then start substituting (conservatively at first) in the next batch.