Rumor had it that the original three would eventually leave - though that was a rumor. I could see the cancer bit being an out for Duffy in case he wanted to leave. That said we are talking about the same guy who got KILLED off originally in "Dallas" but wanted back in so they did that season as a "dream" Bobby's wife had.
I DO however think with JR's storyline as it is they'll finding a way to keep ends open enough should Hagman have to quit the show due to his cancer fight - or worse we have another Jim Davis event and he passes (Note I'm not wishing the worst, this did however happen in the original show). He was undergoing some treatment I think during filming, hence his "traveling."
On the Rebecca/Tommy bit - something tells me one or the other is hit and the twins will be affected by it. This will break the camaraderie undergoing at South Fork between John Ross and Christopher and we're at it again.