Biscuits - Can you hold them overnight ?
I don't know about that. Never tried it. I suspect they might not be quite the same.
One time when I had to have massive amounts of biscuits ready earlier than my normal functional wake-up time, I mixed the dry ingredients and the shortening together, and then in the morning added the milk and cut them out. That only cuts out a few minutes, but it was a few minutes later I could sleep.
I think biscuits are quick and easy enough, personally, that I can't see worrying about something like this. I'd just make them in the morning.
You shouldn't hold biscuits in the refrigerator overnight. You can either do as revsharkie says and go ahead and cut your butter (or whatever fat you're using) into your flour then store that in the fridge along with your pre-measured buttermilk (or milk if you must), then mix them in the morning OR go ahead and cut them out and freeze them IQF style on a baking sheet in the freezer and then store them in a freezer bag or just wrap your sheet pan well in plastic wrap. I know you said that you didn't want to freeze them, but biscuits bake up perfectly, maybe even better than fresh from frozen.
A couple of hours, yes. Overnight, no. Since biscuits get their rise from baking powder and baking soda, they need to be baked fairly quickly, otherwise the rise formed by the leavening settles out and you are left with hardtack. You can make the recipe right up to when you add the liquid to the dry ingredients, toss the dry ingredients into a ziploc bag, keep it in the freezer, and then pick up from there in the morning.