Merge fields belong on the regular page. It would help to see a screen shot of both the regular page a master page. I cannot tell from your description if you even need to do a merge.
ID's native mail merge has no facility to choose a master page based upon a condition. There is a plug-in that can do this from Em Software (InData, I am using the QXP version below). I use it all the time for merging instead of ID's simplistic merging capabilities.
I suspect master page selection could be scripted. But I ain't smart enough.
The below is from a sample file I prepared for someone else. The name tags are a single 4" x 3" page size with bleed. When the company name changes, it applied a new master page (the difference is the color). The screen shot is from after I imposed it.
I've used this facility on real jobs, though, and it works well.
If you post this in the Scripting forum, you should get some help.
Post some screenshots if you can too.
Thanks. Below are my screen shots. There are 300+ pages here which all get different text. Am I understanding if I place that text box on the actual pages vs the master this will work?
2. First Master Page
3. First Page in Layout
4. Excel doc set up (This is just a test sample, there are over 300 different verses)
It should be a fairly straight forward script. Do you have scripting knowledge?
If this is possible with Data Merge, then hopefully someone can help you. If not, if you send me the InDesign File I can see if I can assist with a script.