OTA signal>Diplexer>Dish size>Rain-X
When you read the OTA signal strength, you MUST be on an OTA channel for the receiver to accuratly read the OTA signal. You were probably on a OTA channel when looking at the installation screen, when you saw the higher readings. When you are looking at the screen on a satellite channel, you will see the OTA signal in the teens. Just how MOTO designed it for now.
The diplexer has NOTHING to do with the actual picture quality of the satellite and should not have any real effect on the OTA signal. The insertion loss of a diplexer is no more than 3.5db and that is not going to cause any problems. Now that's not to say a defective diplexer or the wrong one can't cause problems.
Also, as far as the 30" dish working better than the 18" as for rain/snow fade. Yes, the larger dish will allow a higher gain and will not "fade" out as fast during the wet snow/heavy-rain season. Snow build-up on the dish has nothing to do with the dish size either, so you will want to have safe access to the dish to brush off any snow duing heavy storms.
Moisture content in the atmosphere is the cause for loss of signal. FYI: DO NOT SPRAY ANYTHING LIKE RAIN-X OR PAM ON THE DISH, this will NOT solve any problems, only cause more.
Good luck!