While it's possible for mana to despawn (if there's too much of it around, for instance), I believe that mana that ends up in inaccessible areas of the map would be considered "on the ground" and split between player banks at the start of the next phase:
Any mana crystals on the ground at the beginning of a Combat Phase or at the end of a mission will be split among players and deposited directly into their banks.
This is quoted from the Dungeon Defenders Wikia entry for Mana, although I believe it's also a loading screen tip.
EDIT: And to confirm, a passage later in the page answers the second question:
When the limit on the number of mana crystals on the map is reached, older crystals will despawn to make room for new ones. This limit is 30 on mobile versions. Because the limit is based on quantity instead of value, it is possible for older, more valuable crystals to be replaced by newer, less valuable ones. Crystals that despawn are lost and are not deposited into the bank.