That decision is up to Gavin.

I don't think any of the solutions are backwards compatible - all of them require everyone to upgrade to continue using Bitcoin, even if they aren't interested in multisig.  The way I see it, the discussion of backward compatibility really only pertains to the question of what will happen to users that don't upgrade.  The goal is not to ensure they can continue to use their old client (which I understand is true of all multisig proposals including 16 and 17).  Rather, the goal is to ensure that they won't lose coins, and that they won't be victimized by double spends from clever attackers who spend their coins once with the new version and once again with the old clients who can't see the coins are already spent.