hmm, not sure how a fork mounted mudguard is supposed to keep him arse free of mud
For my gravity dropper i just mount the guard above the seat post clamp, infact thats where it goes on a normal seatpost so i dont really have your problem. What rear guard do you have, i think there was once an option for long/short brackets but not sure the most recent one does. There are other guards that might be more suitable SKS, topeak etc. Tho i think you need to give up on the idea if your short and have a 6" bike, surely you wont be able to get off the back of the saddle far enough if there is a guard bolted the seat?