WHY would they let McDonagh and Stralman walk and re-sign Coburn? Girardi's a lot more likely to stay than Coburn, as well, and he wouldn't cost that much at his age and production (but is still better than Coburn). Dotchin's also not worth any more than his current contract; he maybe gets $1m. Same with Paquette (either he keeps his current contract, or he walks and it doesn't even matter). The only way Coburn stays is if Foote, Cernak, Masin, Thomas, Dotchin, and Koekkoek
all turn out to be busts and no team is willing to make a reasonable trade. Actually, McDonagh's already been extended, so letting him walk isn't even an option. Kucherov's also already been extended for $9.5m.
Players like Callahan and Coburn don't have a full NTC, and can block trades only to teams on his 16 team list, so they can be traded. I'd say there are more than 16 teams with enough space to take Coburn or Callahan with a draft sweetener, and there are many teams that could benefit from these players for the short remainder of their contracts. Also, though J.T. Miller had an impressive streak on a line with Stamkos and Kucherov, for the right return, he could be traded (unlike Kucherov or Vasilevskiy, who are obviously untouchable).
Callahan may not have NYR on his no-trade list, as he's from Rochester and used to play for NYR (even if he'd prefer Tampa, if New York isn't on his list, he can't block it). There's also a chance that, when Seattle comes, Tyler Johnson waives his NTC to go in the expansion draft, since he's from Spokane (for free, that would be a bad loss for the Lightning, but considering the upcoming expansion draft, they're going to have to lose
somebody good).
The Lightning can get Karlsson and afford him at $6.5m. The problem is extending Point, Karlsson, and Gourde at the same time, especially as McDonagh and Kucherov's cap hits both go up. Karlsson would have to take a discount (less than $10m for sure), and Point would have to accept a cheap bridge deal (and Yzerman would then have to clear out cap in a few years to pay him a ton of money). I ran through it at
https://www.capfriendly.com/forums/thread/157537 (with Karlsson). They would have to find a third team willing to take Bobby Ryan in exchange for picks/prospects, which would have to be a team not included on Ryan's 10 team no-trade list. But, if the Senators attach Ryan to a Karlsson deal, the return going to Ottawa drops significantly, especially considering the Senators have their hands tied due to Karlsson confirming that he does not intend to re-sign with the Senators under Melnyk.
I ran through one potential stimulation for the Lightning to manage Karlsosn at
https://www.capfriendly.com/forums/thread/157537 . All of those things have to work exactly right, otherwise they end up losing an important asset. If you take away Karlsson's cap hit, you have room to keep a few of the players who were traded (J.T. Miller would be particularly good to keep if possible), and can maintain more prospect depth (so within the next couple of years, they're still deep at forward).
As Yzerman said, they can take Karlsson, but it will require some difficult cap manipulation. But it is possible.
Also, you're overpaying Chiasson.