I am not a fan of diplexing signal with satellite and prefer separate runs especially with reception conditions like yours .
If extra coax runs are not an issue for you, try an appropriate splitter with separate coax runs and go from there.
I use winegard distribution amps when I need to. hda100 or hda 200. you may not need a distribution amplifier with a mast mounted amp.
http://www.winegard.com/offair/amps.htm