A: Discount generally is around 50%. I am assuming you bought Pro during the current Black Friday sale, which would have given you a 50% discount on the yearly plan.

To judge if lifetime is good enough, I would need to know about how much you use Merlin. Lifetime deals come with monthly quotas. It's good as long as it is not empty. But if it ever gets empty, then you'd be out of action. The question you probably need to ask is -- do you use that much, and does the interruption annoy you as much?

If you said Yes to both, you probably are better off with Pro instead of lifetime plan.