Forgot to update: So left the car charging overnight. In the morning, the charger's LED showed only 4/5 lights fully lit and the 5th one still blinking which according to DEFA's manual indicates voltage is between 12.55V - 12.65V and battery status max 80%.
I believe this shouldn't have been the case with a good battery given it was left charging for like 14hr. Start-Stop started working again immediately but after a day's usage and overnight parking the charge status dropped again to something like 58% and SS became again unavailable.
At this point I believe the battery is on borrowed time, and that at one point sooner or later I will walk up to the car and find it unable to start. Since I am in the process of selling the car I will simply make sure the buyer is aware of it but isn't it a rather short time for the battery to go? Just 30 months of ownership? On the other hand I wonder whether that drain incident around a year ago (when the OBDEleven was left working the CANBus all night) took its toll on the battery so maybe not a representative example of how long stock batteries will last on these cars.