How best to cook frozen (homemade, not store-bought) blintzes?
Thaw them longer and be sure they are thawed to the core before cooking; if you made them properly they will not break apart or unfold. 

I wouldn't recommend warming them in the oven because most oven's lowest temp still cooks slightly and deteriorates the dish. 

Once thawed FULLY cook as if it was freshly made (it, of course, will clearly have a minor difference in taste because you froze it and the best course of action would be to make less but if you have excess there's not much you can do besides add fresh ingredients regularly to refresh the old.)