Can I make apple crumble the night before? If so, do I cook it a day ahead & re-heat, or assemble and cook day of? (Will crumble get soggy?)

Good idea! thank you....

I think you'd be fine if you baked it in advance. The part of the crumble that touched the fruit will absorb some of the liquid, but the top will still remain crumbly. I'd probably use more crumb than usual to buffer against the bottom layer that gets wet (but I actually like the fruit soaked crumb). Crumbles are forgiving and always tasty in my opinion.

Thanks...unfortunately, the recipe is classic flour (and one guest has nut allergies). I may have to just prep everything, store separately, and bake it tomorrow.

Depends on the recipe. I would go with a recipe that used rolled oats and ground nuts as opposed to one with just flour. I find that the one with mostly flour get soggy very easily.