I have used the Kenko 2X with the 60 MM Micro and it works quite well, but as stated your working distance at full magnification will be quite short. This will make strobe positioning difficult and also cause your subject to retreat!
When shooting 2:1 you will need a nice bright spotting light to compensate for the light loss due to the magnification
Regarding auto focus, At 2:1 I suggest you try to avoid using it due to the very limited DOF and low light levels. If the camera can not "lock" in on the focus, the shutter will not release. This will drive you nuts and very likely lead to many lost opportunities!
If you do not have manual focus capability with your housing AND you plan to shot a specific subject, I suggest shooting in full manual focus mode (camera servo and Lens) and presetting the focus distance so that you can fit your subject in the frame. You will have to move the camera to achieve focus, but you will have the ability to release the shutter when you desire. The focus indicator Dot located in the far bottom left portion of the viewfinder will still function, therefore you can use it to determine when to press the shutter button. You may get a lot of soft images, but your chances of getting keepers is greatly increased. I do suggest that you try give this technique a try in your local area or even in a pool before you apply it on a expensive dive trip. 2:1 is not easy, but with practice and a LOT of patients wonderful results are possible. For good results you should plan to spend most of if not all of the dive on a single subject.
In my opinion, you should get the 105, as it is very useful for super macro. The 105 may actually aid in you success with the Pigmy Seahorse ( they tend to be a bit shy). Check Ebay, they are quite common and very reasonable.
Kindest Regards
Chris Bangs