what exactly do you want to connect to QML? single button clicks form the widget UI or something like that would be not that easy but you should be able to start a QML window from a UI widgets or vise versa but accessing the items of the component can of course be archived from c++ in both cases..
I also want to single click, but note: you only need the double-clicks in the Projects window. So I usually open all the .cpp and .h files I need in advance, then I split the sidebar, and select Open Documents in the other window. And in the Open Documents window you can use single-clicks :-)
for the registry keys, you can add an operation for GlobalConfig in the install script.
component.addOperation("GlobalConfig","HKEY_LOCAL_MACHINE\SOFTWARE\MYCOMPANY\MYAPP","TESTKEY","42");
Update: I think I could verify two bugs in Doxygen. I posted them here https://sourceforge.net/p/doxygen/discussion/130996/thread/f30fd882/ and here https://sourceforge.net/p/doxygen/discussion/130996/thread/f30fd882/ . In the latter link, I explained how to fix this problem manually... So it's not a problem of Qt Assistant per se.
Nope.
I think I have found just about every reference to using stylesheets for QtCreator on the web. I started with the one you referenced above to get to where I am. I cannot figure out what the handle is for the text in the Locals and Expressions pane.