Quite possibly, the number one mistake of novice paella makers is stirring the rice!
Paella is not a risotto. So don't stir it. The rice should be loose and pearl-like in texture. Not gluggy and creamy.
Which is why in fact, I hate serving paella anytime after about 23 minutes from completion - it just starts getting gluggy. Not to say it isn't still amazingly tasty. But i'm fussy.
For the Spanish rice dice to be cooked evenly without stirring requires a bit of chemistry and physics. The right amount of boiling stock, the right amount of heat, the right pan, and the right level (flat!).
As for leftover paella, I like to pan fry it rather than microwave it. I rarely microwave anything as it is, but I find frying it loosens it up a bit. And never, ever, freeze paella Spanish tapas. That is all.