GoInstant developer here. First thank you for your kind words and thoughtful comment!

We did focus purely on text synchronization as the first step so you are correct cursor positions may be in the wrong spot if you are just moving the cursor around while conflicting text is being inserted/deleted. We do also use a post OT heuristic to place the cursor as the end of a text edit though so the cursor while typing should still look natural. Since cursor position is not included in OT I'm not sure I follow your example where <p>abcd|ef</p> becomes <p>abc123d|</p> though.

This was a tradeoff we made to launch earlier but more core cursor sync support is definitely possible in the future.