Is Source Control possible for Robohelp 2019 Classic?

Ahh. That is a different question. While I'm unsure how exactly storing the project in sharepoint without source control works, you may have run into the well known "corrupted project when run on a network location" issue. 

Version and source control are slightly different, but close enough that the terminology doesn't matter for our purposes. While you don't say what version of sharepoint you're using, Classic I think only supports version/source control in on premises Sharepoint servers, not Sharepoint Online. https://helpx.adobe.com/au/robohelp/using/rh_system_requirements.html

If you had version history enabled in sharepoint, perhaps you can get a copy of all files from before the problem started. I'd try getting the files from before the issue and saving them outside the sharepoint folders, say c:\recoveredproject and see if everything works when opening fresh from there.

To set up source control using sharepoint, I think you would Add to Version Control, select the sharepoint connector option and fill in the dialog box. The first field would be the address of the sharepoint server where for the document library. In the RH2017 help I have it says you need to add the project to a blank folder inside the document library. It implies you can create a new folder within the dialog box but I've never tried myself, so you might need to set that up manually in sharepoint.

If you can't get back a known good version, then we'll need to work through the issues. The first step would be moving the files outside the sharepoint folders, similar to my suggestion for the copies of known good files above.

I'd recommend not having multiple authors until we get the project back into a working state and work out what sharing method will work for you either temporarily or on a more permanent basis.

If you click the little blue help icon in the top right corner of the Classic application menu bar, that will allow you to open a chm file with help more relevant for Classic. See if the Version Control section of the Manage projects topic helps. Peter's site might also contain information about version control in Classic. Post back if you still have questions.