Ideally they would trade for one or more legit top defensemen.
Since that is unlikely, here's the best solution I see:
1. Let Q walk.
2. Get rid of Ericsson by whatever means are necessary.
These two points are critical, since you can't bring in better defensemen without making room.
3. Rotate Marchenko, XO, Sproul, and Jensen through the lineup. Play them in all situations and find out who is good at what.
4. Consider bringing up Russo and giving him a big role to see if he can handle it, or if he's just an AHL phenom.
Since the best way to fix the defense is internally and these are the best guys we have, it's time to find out if they're going to be keepers and whether any of them can be star level players.
5. Consider picking up one or two lottery tickets in free agency. Can't spend a lot, so attack the problem with volume. A guy like Jamie McBain might make sense here. 28, right shot, has pretty good fancy stats, shows some offensive ability, would be cheap. Carry 9 defensemen for a while if need be. Don't let convention get in the way of assessment.
6. Be prepared for some ugly mistakes while you're sorting out who to keep and who to dump. This will be painful, but considering we're ridding ourselves of Ericsson's ugly mistakes in this scenario, it should be a wash.
7. Kronwall to the third pair and the 2nd PP, and off the PK. Stop pretending it's 2008. It's not.
8. Change from low event to aggressive transition hockey. Guys like Green and Smith and Sproul should thrive under such a system, and low event hockey is boring.
9. Understand that you're not going to win the Cup in 2017. There is absolutely nothing you can do that would enable you to win next year. Take the opportunity to do a full audit of your organizational defense. Slough off the dead skin and move forward.