I use a pastry cream recipe that firms up quite a bit...almost rubbery. In using it to fill eclairs and the like, I usually fold in some whipped cream before filling. But it's veeery hard to get the lumps out. (What was perfectly smooth pastry cream when first made and cooled gets lumpy when I try to remix it.) So...
1. When you fill a cake with pastry cream, do you use it "straight"?
2. Any tips for how to soften it back up after refrigeration without having it clump?
Put it in your stand mixer with the paddle attachment and beat the living daylights out of it. Then you'll be able to fold the whipped cream in with no problem!
I only use straight up pastry cream for some recipes (but it's usually heavily flavored). Most often I'll turn it into a creme mousseline, or a creme legere (which is what you are making--this is also what I use for most of my cream pie fillings). I very rarely use pastry cream as a filling on its own.