Thank you.Well do this at your own risk, but the best I could come up with is download RPGMakerSaveEdit, set variable 239 DragonStage to 4, and uncheck switch 445 Gemstone Received. If you've already done the temple quest then I'd recommend using a save editor to remove the gemstone from your inventory after getting it to prevent issues there. Hopefully that works for you. Also make sure you are OUTSIDE of the cave before making the changes, the event triggers on crossing a threshold inside the cave.
Tried installing .NET 4.5.2 and VC++ 2017 runtime with winetricks and couldn't get RPGMakerSaveEdit working in Wine. Will have to try using my wife's laptop.