The Well monster spawn is based on the ITEM LEVEL you put into it, so if you have a level 55 white item, the monster will be at 55 (Well Spawn).
Well Durability is RANDOM, it is not based on the Map level, it is pre-determined when spawned.
You can put either 1 item in and poof, or 100 items and it still remains, you just ran out of whites and nothing on MP to purchase, you can not whistle or spin in a map for new monsters to spawn for whites, so you make new game... I have done this before, my vault got emptied out and the whites I put on MP were all used up in a Tier I Map with +1 Well.
Awesome, good to know, appreciate the info!