Alexander may still be your best option as a keeper under those rules, depending on whether or not Seattle signs or drafts significant competition. It's entirely possible that he can get back healthy this offseason and get back on track. By "on track", I don't mean top 5 RB, but I could see a scenario where he is a #2/#3 fantasy RB, that can get you decent production with certain matchups.
It sounds like with these rules, that's the situation that everyone is going to be in. I would do everything I could to make sure that I wasn't keeping Stallworth. He has a ton of potential, but he's been in the league long enough that he would have realized by now if it was ever going to happen.
My advice is to wait as long as possible, hopefully you can wait until after the draft, and see what the situation is in the Seattle backfield. Even with his two crappy seasons in a row, I would rather start off a season with him on my roster than Stallworth.
No one's going to give him a chance to be a #1 WR. Even if he goes to Tennessee, he will still be the #2 WR behind Vince's imaginary WR that he likes to throw to all the time. Just kidding. But really, he might be #1 on the depth chart, but if he can't play worth a crap, who cares? All that means is that the rest of the guys there are comatose.