I think you are on the right path, and will need to go through EDL-mode.
If the phone were turned off, the battery of a phone usually last for at the least a year if fully charged, without depleting it self.
But if it was in a state where it turned it self off, and then was left for months, it might well be problematic. I do not know if there are any tricks to charge the battery in that state without opening the phone and charging the battery directly, by a circuitry bypassing the charging-logic (do not give the battery 5V directly).
Many phones by default comes with a silly mode