Pages Data Merge

Pages v5 through the current Pages v8.1 never have had built-in Data merge. The retail version of Pages '09 v4.3 had built-in Mail and Data Merge. None of the preceding will do what you claim has happened, so what specifically did you download, and from where? It sounds like malware masquerading in a trial version of iWork '09, but lets be specific what it was.

I downloaded and used the Pages Merge tool last evening. Got it directly from the iworkautomation download site. It did nothing other than what it was written to do — perform data merge from a Numbers spreadsheet into a Pages v8.1 document. No other collateral effects as you originally described were observed, nor are they reproducible here.

If on the otherhand, you downloaded the Pages Merge Tool from any other location, then an opportunity for bad actors to add malware into the tool may have existed, and why I always recommend software be directly downloaded from the original developer site.

I don't know how to proceed with this. I used the same Pages Merge application and cannot reproduce the calamity that occurred for you. I am inclined to believe that it was something that you (or malware) did prior to downloading, and the use of this application.

The Pages Data Merge application works fine without any side effects when run with Pages v8.1, and Numbers v6.1 on macOS 10.14.6 (18G87). The Pages and Numbers documents must be previously saved documents and opened when the Merge application is run. It is a third-party application and the author of it is the sole source of support.

If you are attempting to use these applications on a beta release of Catalina, you are getting ahead of the software, and it may break. I am bound by the terms of the Apple Support Communities to not discuss Apple beta products, if that is the case.