[Nerwen]

1. I don't think Nimloth was exactly a cherry-tree– it merely resembled one in form. Thus, any information about the rarity or otherwise of actual cherries in M-e doesn't apply. While most of the plants Tolkien mentions can be related to actual species, I doubt all of them are literally those species– not the magical ones, anyway. Nimloth, apart from being extraordinarily long-lived, was ultimately a copy of Telperion, which glowed with its own light. So, not your average cherry.

2. We are told in "Return of the King" that "the fruit of the Tree comes seldom to ripeness (...) if ever a fruit ripens, it should be planted, lest the line die out of the world." So no, I doubt Nimloth jam was ever on the menu.