So the program works as designed, until you install a third party extension, then it loses some of it's base functionality that is replaced with a listing bearing the name of the third party developer?
I'm no rocket surgeon, but...
I'm guessing "something changed" in the program since the extension was written and that particular extension, whichever one it is, hasn't been updated by the developer to reflect the new menu system yet.
Adobe has been modifying the menu system slightly with just about every release under the CC banner.
You can test if that's the case by installing an older version of DW, then installing your extension into that version. As a CC subscriber, you can have all 5 releases of DW installed on the same machines at the same time (CS6 Cloud, CC, CC2014.x, CC2015.x and CC2017.x). They won't run concurrently, but it would possibly allow you to run an older version with your extension, save and close the older version, then open in a newer version to use the latest features/functions.
To get an older version installed...
1. Open the CC Desktop App
2. Click the Apps tab
3. Scroll down to the Find Additional Apps bar
4. On the right side of that bar is a blue link, click it
5. Choose View Previous Versions (the bottom most option in the pop up menu)
6. That will populate the Find Additional Apps section with Install icons, click DW's
7. Choose the earlier version you want to test (I like CC2015 personally) and install
From there on out, you'll be able to run either version.
When installing new versions in the future, make sure to click the Advanced Options link in the installer dialogue. That will let you turn off the checkbox for "Remove Older Versions" allowing you to keep any older full version you have installed. Unfortunately, dot releases will overwrite each other even with that option unchecked.