Yesterday, I found mine still reported 100% on the day that the sensor stopped working and the battery measured 2.38V. That is about as dead as a 3V lithium battery is going to get.
As noted, if the device provides garbage data, then there is not much that can be done to massage it into good information within HE. Overall, I don't have a great solution. I am trying Device Activity Check to alert me to inactive sensors. Even that is tough to balance nuisance alerts against waiting so long that you just notice rules failing.
For a fixed solution such as the sump pump that you mentioned, I would consider also hard wiring a 3V supply or "battery eliminator". It sounds like you are already handy with a soldering iron.