To your notes and questions:
Why are you using a custom version of cocos2d-x? What features did we lack for you?
The main reason I created a branch was after I realized that desktop support wasn't a priority for cocos2d-x. A cocos2d-x team member made that pretty clear in this github issue - which remains unplanned. After creating our branch, we've slowly been adding more features and fixes, most of which have yet to be implemented in the latest version of cocos2d-x. You can see the commit log to our branch here to get an idea of the type of stuff we've been adding. To summarize, its included desktop related bugs, culling when in 3D mode, camera adjustment, and controller support.
You might want to consider moving to v3.8 when it comes out. This is a stability/bug fix/polish features release. We should be about very soon.
As we approach beta, I'm trying to lock down our code base, so I'm not sure I'll get the chance to merge the latest from v3.8. Things have actually been pretty stable.
Can you share with everyone how you are using Cocos2d-x and Steam? Most folks here seem to be developing mobile games so using Cocos2d-x on Steam might be an interesting read for a lot of us.
Here's a post I wrote a few months back. It covers all of the different frameworks and tools that we use to develop our game. Nothing has changed in our dev environment since then.