Can I make the batter and the filling a few hours ahead and store in the fridge?

Absolutely. Filled blitzes also freeze well.

I think you can pretty much refrigerate blintzes at any step- prepared batter and filling, prepared crepes and filling, prepared filled and folded blintzes ready for the final crisping in butter.

Sure. I usually refrigerate my crepe batter for a couple of hours before using, because my mother did it that way (I think she thought it would relax the gluten?). And the filling would be fine refrigerated too.