The O ring goes over the starter motor and holds the FOD guard on. It does an adequate job and I don't know anyone who has had problems with this method. The high-temp silicon probably won't cause any problems (other than getting goo on your engine), but its unnecessary...
I think that the engine mounting rails are spaced for a 120mm ducted fan and thus are too far apart for the smaller turbines that are suitable for this model.
Bob