I am in the same situation you are, sort of. My choice is only Kelley or Bell.
I have Kelley in my flex currently. However, all the FF experts are saying to bench Kelley this week due to NO being decent against the run.
ARI is weak against RBs this year, but I dont know if Bell will receive a full work load this week and I absolutely, positively do not trust in the #2 coach he has.
For you, keep Jefferson in. SEA defense is too good of a match up to not play Jefferson.