This makes it difficult to reason about, for example, the task of scanning through the database and deleting keys or values according to some criterion. 

What does MDB_NEXT mean on a cursor after deletion? Will it return the next element, or should I use MDB_GET_CURRENT for the next element? Or will these operations return in error, and I should use MDB_SET_RANGE with the deleted key?

Similarly, what happens to every other cursor pointing to the same key? I see you have some code that touches them, but no documentation to tell the user's what is happening.

This needs to be documented. Iterator invalidation is painful in C++ where it is thoroughly documented. When it's reduced to guesswork or testing without a clear indication of intended behavior, it's worse.