Would it be possible to create a mod that adds an elevator to the skull cavern, like how there's an elevator in the mines?
I don't imagine so, but there is a mod to let you save floor progress. http://www.nexusmods.com/stardewvalley/mods/175/?
*false The floor level for the mines (Both of them are a single system) is stored as a signed integer, this means the maximum floor you can reach before overflow occurs is floor 2 147 483 646 So massive, but not endless