Shattenkirk rejected a sign and trade from the Oilers and the Lightning so I'm guessing he knows where he is going as a UFA.
No way the Leafs would pay this price for a rental.

Leafs are on his list and he'd sign there. as for the trade Kapanen is on the leafs "love" list meaning pretty much untouchable.

Shattenkirk's agent gave a list of at least 6 teams he'd resign with as far back as before the draft. Edmonton wasn't on the list but tried anyway. Obviously TB & Toronto are on the list but Shattenkirk still decided offers from both.

Here's my guess and it's 100% guess.

Army was up front with Shattenkirk's agent letting him know he's on the trade block etc.. Army expressed it could be beneficial to both parties if a sign and trade deal were to happen. Shattenkirk agrees to give Army a list to be a good employee but never has any intention of helping Army & the Blues out by actually following through with a sign and trade deal.

Army spends months trying to procure a deal. Every deal offered Shattenkirk doesn't agree to.

Army finally gets frustrated enough to start leaking some details on proposed trades to put some egg on Shattenkirk.

I think Shattenkirk gets traded for rental prices and goes to UFA like he's wanted to do all along.

That's an overpayment... Especially as a rental

It's a ridiculous overpayment for rental, especially for a rebuilding team. Of course Shattenkirk's agent is going to say they are willing to sign with multiple teams. What else is going to say, I'm only signing with Boston and the NYR areas teams, doesn't that limit his negotiating power?
IMO the Blues motivated to make the playoffs and more so unless the Blues get immediate help, they aren't that motivated to make a deal. Again, the don't think the Leafs are that interested in him long term. Let someone else overpay and the Leafs will acquire other guys.