Can I re-heat Ricottta that hasn't seperated into whey fully yet?
Yes, you should be able to, if it's still liquid.  The worst that will happen is that you'll end up with a more dry, farmers-cheese curd, but that will still be fine in ravioli.

Incidentally, this is why a lot of folks use granulated citric acid instead of lemon juice.  Juice acidity can vary quite a bit.  Otherwise your quantities look correct.