Regarding the old autosave, it is maybe related to the problem I have reported few months ago here:
In certain scenarios it seems GDevelop does not create autosave regularly every time the preview is launched.
Regarding the corrupted project file, I’m afraid nothing can be done. It is supposed to be a simple JSON file that any text editor should be able to display especially VS Code. If not even VS Code can open it, then it is very unlikely anyone can recover the data. In the future, create a backup every day and after every major milestone and store the backup in a different folder, USB, cloud.