Hi, Well let's take one question at a time.
1. My guess on your hydrangea is that it's a Lacecap variety. Lacecap hydrangea flowers only have petals that open on the perimenter of each flower. This is how they grow naturally.
2. The best time to prune grapes is in late winter or early spring before they break dormancy.
3. Doing a graft on a plum tree is a bit complicated to describe in an email. I suggest you do a Google search on fruit tree grafting. It's not particularly difficult, but requires a lot more detail than I can give in this answer box.