Hi Craig,
As I suspected, the problem was in how your code was structured. I run into the same issues when I started using the EE. Look at the attached file.
I recognize that not every change that I made may be necessary, I made changes while I was trying to make the snippet work (the file as you sent it wouldn't compile at all for me), and currently don't have time to strip it to the minimum. But I hope that at least a working example will be useful to you.
The basic problem is that you were placing many overrides to the same property in the same moment, which caused problems: the EE can't overwrite an override that is already present in the music input. However, if you move your general override (named \gossecDynamics in your file) in the layout settings, then there is no conflict since there are not two overrides present at the same time, and the latter (created by the EE) overwrites the general one. You should have gotten warnings for this I think? Do you have the Edition Engraver on the latest version?
Hope you find this useful,
Stéfano