FMP 14 Advanced installation problem

I decided to install the Mac Mojave OS (10.14)...BIG mistake! I found out too late that my FMP 14 files would no longer work. I reinstalled Mac OS 10.13.6, only to find that my FMP application files were no longer in my Applications folder.

I dug out my FMP 11 Advanced, and my FMP 14 Advanced updater and reinstalled them. This seemed to go smoothly but, when I tried to open any of my FMP 14 files, I was met with two error messages as follows:

"The document “11/14 MASTER copy.fmp.12 could not be opened. Filemaker Pro Advanced cannot open files in the “Filemaker Pro Advanced.app document” format."

and

"This application has been installed incorrectly or modified by another program. Please run the installer to get a fresh copy of the application after determining the cause."

I have twice reinstalled FMP 11 Advanced, followed by FMP 14 Advanced updater, and FMP 14.0.6 updater... all to no avail.

I find that I can open FMP 14, and then open my files from within the app, but this is inconvenient, and I am afraid that I may run into trouble sending files to my customer with a possible problem like this.

Can anyone tell me what to do now?