Workaround is probably to download Process Monitor from SysInternals/Microsoft. This tool will let you see what EGS tries to find as it launches in terms of registry locations and files.
Most likely it is failing to find a file or registry key that tells EGS that you have game X installed in folder Y.
Once you recreate this key, you should be good to go (i.e. restarting EGS and it should find the folder/reg key).
Sometimes you’ll need to recreate more than one.
All of the app stores have issues like this in my experience. For example EA Downloader might let you pick the folder the game resides in, but it still runs a verification on ALL files AND then does a “touchup.exe” which basically reinstalls the game/components, so, get ready to get Visual c++ and directX redistributables run for the 21000th time.
Turns out the .manifest files are the installation settings for each game you have installed.
So if someone sends you their manifest file for Rebel Galaxy (they can try replacing their SessionID with 0s before sending) it might work (The installationGUID in the file is the same as the file name).
You’ll just have to edit it to pick the right path.