I made this recipe for dark chocolate ice cream. I did a double recipe, using 3 egg yolks + 1/2 cup cream instead of 12 egg yolks, and I left the cocoa nibs in until right before  churning to try to extract as much flavor as possible.   The taste was excellent but the ice cream had a somewhat gritty/sandy texture. I am fairly sure that the sugar was all dissolved, as I cooked the custard slowly and whisked constantly. I think the issue is either the cocoa nibs or the cocoa powder. I was originally planning to leave the nibs in for a crunch, but I tasted one after cooking the custard and it was not good - starchy and bland. Why did my ice cream turn out gritty? 
Sorry, no.   Whipping cream that has been frozen is perfectly fine to use in many dishes, but it won't whip.  As far as I know, this is irreversible, so no, you can't make the cream "whipable". If you are very lucky, you will get it to a sort of semi-stiff stage, but nothing that would be suitable for cake decorating or similar.    Not part of your question, but possibly interesting for next time: Whipped cream can be frozen, so if you have leftovers, you could pipe little tufts or similar, freze them and use them to decorate desserts. 