Go get whatever conversion you can get with the new change to the FARs before it goes away. Worst case scenario, you were never an IP in a military multi engine aircraft with no centerline thrust restriction, so you would get a CFI-Multi Engine land restricted to centerline thrust and have instrument privileges.
Each successive instructor check I took got easier. The first one was the worst, about an eight hour ground eval from a designated examiner plus the flight. The second one was only about an hour of one instructor topic since on the previous, I demonstrated all the other knowlege of the FARs, Fundamentals of Instruction, and my lesson plans.
Even though it seems as if you were doing them backwards order from most folks, walking into the single engine CFI checkride with another instructor ticket should minimize the pain. Do not be unprepared, know what it takes to get a zero time civilian to a Rec/Sport/PPL ticket and know the Practical Test Standards.
If I had to guess, I would say you would still have about a two hour ground eval, just because the of all the Part 61, 67 and 91 questions that would come up. The flying part will be relatively easy, if you get with someone who can teach you to teach the civilian commercial manuevers to the PTS.
You would also have to take a commercial single engine checkride, because you have to commercial privileges on your single engine ticket to be eligible for a CFI-Airplane Single Engine. So your commercial check would be practice for your CFI check ride.
That is my best hack at it, but I put the caveat on that I have not done the civilian side of the house in a long time.