Are you using and Eakin Seal with the mouldable? I found that the two, together, helped prevent leaks the best. What size mouldable wafer did you purchase? I used the small and was able to open it a little bigger than my stoma, put on the Eakin(sized to my stoma size, so it would be smaller than the opening), and apply. Within minutes, the part that I moulded back "turtlenecked" my stoma to the perfect size.
As far as sleeping on your stomach, go for it as long as it doesn't hurt. I don't need pillows anymore, but that took time. When I go to the beach and lie on my stomach, I use and extra beach towel to make some extra room for my stoma. You could do the same for your massage...
As for your seatbelt question, I usually make that portion go below my appliance or if I am not driving, I hold the bottom to give it a little slack.
I hope your leaks stop soon!!