Hi Kory and Tim. First, my apologies for my frustrated rant. Below I have outlined what has occurred as best that I can determine.
1. I did not know of the 'decision tree' that was shown to me by Tim. So, I could not get the required Microsoft Access Database Engine 2016 files to install as described when upgrading, so I then had our IT support company install the Access DB 2016.exe for me. They got it installed and the ArcGIS Pro projects would open (those with and those without Excel files in them). I do not know if that means the files were installed silently or not. The Excel application on my machine indicates it was installed Click to Run.
2. Microsoft Access Database Engine 2016 is currently installed on the PC.
3. My Aus support case was Esri Case #03129769 - ArcGIS Pro 3.0 Corrupt Upgraded Project Files?
The responses (and support assistance) were always generally helpful but I/we have never been able to resolve the issues that still prevent opening some projects.
4. Should I uninstall Arc and reinstall it? Going back to 2.9 is not an option now that I have already upgraded most projects to 3.0.1. But I think that is all I can do from this point? I haven't done this already because I am paranoid I will not be able to open any of the projects at all, i.e. break something else. However, I think your meaning is to rollback to 2.9 and open the older projects that don't currently open in 3.0 and then somehow save them so they open in 3.0?
5. I have never used project packages. I will see if I can do that and send one to you.
Thanks.