You can have multiple qml files open by either using the “Split” functionality, or since Qt Creator 3 you can also open as many qml files simultaneously as you like with “Window / Open In New Window”.
Qt Creator in general is very powerful to customize: you can define all the shortcuts as you like, change the position of the panels, and much more. We have mentioned the most useful shortcuts in our entry tutorial here: http://v-play.net/doc/vplay-qml-and-qtcreator-usage.html#qt-creator-tips-shortcuts
If you have ideas for interface improvements, you can either implement them by yourself as Qt Creator is open source, or get in touch with the developers of Qt Creator in the qt-project forums or Qt Developer Mailing List.
Personally I find Qt Creator the best IDE (after using Visual Studio, Eclipse and XCode for several years each) which is constantly updated and improved, although getting familiar and highly productive with it might take a few days/weeks – but this is just a personal thought and I don’t intend to start a war on that opinion. 😉