Thank you, @martijnr!
All that sounds good to me. I don't feel particularly strongly one way or another about session. I think in 3, we'll want pyxform to accept timeStart, timeEnd, etc but convert them to actions/events. As we've done with other similar changes, I'd like to ask that 3 be done a while after 1, 2, 4 and 5 so that users of older clients can have time to update before forms start using new features. This is particularly sensitive because users rely on these values being populated.
As part of this, could we also consider adding more metadata elements that users have asked for? The big one I can think of is some kind of client identifier/user agent (e.g. "Enketo 7 Windows 3.1 Firefox", "Collect 1.21.1 Android 7.1").