They all have their pros and cons. If you move Rielly out suddenly their offence from the back-end becomes deplorable. He's not perfect and he requires a lot of responsibility from the coach to get him in the right spots where he isn't exposed (and ideally not hemmed in his own zone) but he is still a huge net positive, on a fair contract and not old. Muzzin is the declining asset, but he has also been their best d for several years so it hurts to lose that. Gio of course doesn't replace what Muzzin has brought to this team and I don't think you're winning the cup with him as the 3/4. Maybe if they had a Hedman type, but they're gonna have to make up for their lack of Hedman with solid depth and no real weak spots.
Sandin... Not sure about him, but I would hate to let him go before he's given a real shot in the top 4. I think they've gotta do it next year and if it doesn't work out you make another Gio trade at the deadline. Unfortunately this just means you've gotta let Muzzin go if you can. Full NTC so he has to be willing and the options may be limited, but I think it has to be the move.