In reply to Simon R Jones.

WP Migrate DB (or the Pro version) does not merge databases. It simply overwrites the targeted tables with the source tables. It is very elegant and works well for what it does. One really handy thing it does is to automatically replace URLs as needed from one environment to the other.
But it does not solve what seems to be the Holy Grail of multi-environment collaborative WP development...which at least for me is the biggest pain point of multi-environment developing.

This multi config file thing looks very clever and should at least help with part of the pain anyway! Thanks!