Personally, I think the Castle runs/Beau Biden didn’t scenario was much simpler than any arranged deal for Castle to win and then bug out later.
The 2010 election was looking like a bloodbath for Democrats that even a state like Delaware couldn’t escape. (Remember that we had already taken Massachusetts with Scott Brown’s victory by then, so for a time it looked like we could win almost anywhere) Coupled with the fact that Castle was the state’s at-large congressman and former two-term governor, and the seat looked like too much of a gamble for even the son of Biden to take on.
I also find it more plausible that Castle would have tried to occupy the Senate seat until he went out feet first. Delaware isn’t far from D.C. He could have kept going there even with increasing age and health problems (he wasn’t a spring chicken when he ran for the Senate and had suffered two strokes in 2006). Look at how many senators recently that refused to vacate the chamber even in light of bad or terminal health and eventually died: Byrd, Kennedy, Inouye, Lautenberg, and almost certainly McCain to come. These guys don’t let go of power easily. I doubt Castle would have done so either.
Castle would’ve left early for that cushy reward in the Zero regime, or he simply would’ve switched parties had he attempted to stay in the Senate. He was more than comfortable with the Democrats.