No, it is not a WPMU.
No, it is not domain mapped. It was formerly hosted by my servers but I just recently moved the domain and hosting to Dream Host vice local due to connectivity issues with TWBC.
English site, no configuration to language.
There are no plugins installed that interact with the visual editor.
Recent changes to the site configuration include:
SSL to admin area (via config)
Changing the wp-content directory name
Specifically defining the following constants in wp-config:
I can't think of any other major changes I've made that could possibly have affected it. The same plugins I have installed now (and I would even daresay the same versions of said plugins) were there before and everything was working fine.