Seems like replacing this assembly isn't that much harder or more expensive than using one of these couplings, and it is a nice, relatively simple, first project since it's just replacing what's already there. Replacing the assembly would also possibly give the OP the chance to convert that existing s-trap into a p-trap-- that's a bigger siphon concern to me than the (future) a/c trap.
For the a/c, as long as it's a 2" p-trap I think it should be safe from siphoning. The catch is joining abs and pvc. If it were me, I'd set it up as an air gap drain (like a washing machine drain): build in a trap between the wye and a new 12" vertical 2" drain. Then just sleeve a few inches of pvc in the top of the abs, eliminating the need for a reducer and any cementing. The only (tiny) concern in my mind would be that the trap would evaporate dry in the winter.