Your thinking was correct, the modification date of the file should tell you when it was last modified and how much data is possibly retained. So if you have a file with a modification data and an Autosave with a later modification date, then obviously the Autosave version is the later that has more data left.
Open the Autosave file and save it under a new name.
Then you will have it and you can check whether it contains the info you need.
It is good practice by the way to just do CTRL+S to hard-save the file, something you apparently have not done in that 6 hour period. But that is for future.
The Exclusive access message comes up because of the LCK file (lock file). The LCK file tells ArchiCAD that someone have is already using the file.
In case of a crash, the program does not have the opportunity to delete the LCK file when you close the PLN file, so it stays there and gives you this message.
But if you were using the PLN file, then it is of no consequence to open the file with exclusive access.
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