Ctrl+2 and Ctrl+3 select the Edit and Design modes respectively: no mouse required.
Ctrl-Tab can quickly cycle to another open source file (away from the UI XML, which I agree is largely pointless).

You can press F1 with the cursor over a keyword to open help side-by-side with the source. Ctrl-6 switches to the full-blown help view. Assistant is still a separate application if you wish to run it that way. A Start menu icon is not created by default: you have to choose one from the potentially many Qt versions you have installed in parallel and add the documentation to it.