As far as trading up from #16, after the 9th or 10th player the talent level seems to be roughly level until about #18. So, depending upon the trade partner whether a player drops, and what other teams want (both in talent and in trade assets), moving up to #10ish may be possible. If GMs of other teams basically don't see much differentiation in talent between 10 to 16, or, they expect a good player for their team to still be around at 16, then moving down would allow them to get a player they like and pick up an additional draft pick or two.
The question for Ainge is whether a player who might be at the 10 to 12 range is really someone he wants. Possibilities might be Luwawu, Labissiere, or perhaps Deyanta Davis. These players seem to be high risk, high yield sorts and with versatility to fit a variety of roles for Stevens. Ainge might see them as the sort of players worth gambling on.