A couple of thoughts:
A) Goya's "Base Elements" is a hugely useful tool for analysing the DDR of a multi-file system. Check it out at http://www.goya.com.au/baseelements
B) The following "order of operations" makes for smoother transitions:
- Create layouts with names taken from source files into each core file, using existing TOs (of external tables) as base table for each layout, matching TO names.
- Copy/replicate scripts into each core file, and point them to use the layouts created in 1 above.
- Copy layout content (i.e. field objects, buttons etc) into each core file.
- Import tables and fields into each core file, and repoint existing TOs to internal tables. Repoint external references to new file/table.
- Migrate data from live system and make available for testing.
- After testing and debug, clean out test data and migrate live data in.
Note 1: "TO" is "Table Occurrance"
Note 2: if the layout and TO names in the destination file match those in the source file, scripts will mostly import without problem. Once layouts and scripts exist, copying the layout content happens more smoothly - once the layout parts (Header, Body, Sub-Summaries etc) exist in the same vertical sizes a copy/paste usually does the trick.
C) If you are using Base Elements, it becomes much easier to "clean up after yourself" before data migration and testing. E.g. any buttons that lose their script connection in the copy/paste will be flagged as errors and therefore easy to find.
Consider an out-of-house consultant that has experience to combine the files into a single file for you?