At this point I'm suspecting you have a dead battery either because of the corrosion damage and/or being left for so long in sleep mode.
While the USB-C power checker will help in seeing if the charging logic is even working it won't fix the dead battery. I would just bit the bullet here getting a replacement battery [product|IF455-022], then carefully plug it in (leaving the original one in place) so you can check the charging logic using this gem of an app! [https://www.coconut-flavour.com/coconutbattery/|CoconutBattery] take a snapshot of the apps main window and post it here for us to see [guide|21499] it will show you what your USB-C power adapter is doing. As the shipped battery will only be partially charged your system should be charging it. If not you've proved you have a logic board issue besides the battery.