StartTabId will work with any level setting, however, the level setting does determine what nodes will display. 

Level settings of parent and child do not specify that sibling nodes will be displayed, the others do.  So if you use a rootonly="true" level="parent" starttabid="123" it should produce the desired output.

My module should work with DNN 3.x, though I have not specifically tested it with 3.2

It is not a matter of defaulting starttabid a certain way.  It is passed as whatever value you set it to the underlying framework.  It is a matter of what nodes the underlying framework is told to show (i.e. include siblings or not). 

The example I provided does work... I am not sure why you are saying it doesn't....  Are you specifying a real tabid?