Solved my problem on my own. I also tried selecting the sketch entities with SelectByID2, because it advised to use that instead of the Select4 methods in ISketchSegment, ISketchPoint, etc... but I got an exception when I passed 'null' as the callout argument... I don't know why, looks like a bug to me. But I looked again at it's documentation, and I saw at the bottom that I could use
I really wish that information would have been also in the Select4 method's documentation page, that would have saved me a lot of head scratching. What use is the Select4 method then, if it just doesn't work and the documentation advises NOT to use it, but not in it's own doc page? sigh...
anyhoo, here is the fix, for anyone struggling with similar problems:
SketchManager skm = SWIntegration.Instance.getModel().SketchManager;
skm.AddToDB = true;
SketchSegment ss = skm.CreateLine(0, 0, 0, 1, 1, 0);
SketchSegment ss2 = skm.CreateLine(0, 0, 0, 1, 2, 0);
SelectData sd = SWIntegration.Instance.getSelectData();
SelectionMgr sm = SWIntegration.Instance.getModel().SelectionManager;
DispatchWrapper dp = new DispatchWrapper;
dp = new DispatchWrapper(ss);
there you go.