Barbashev's a UFA after this year. Toronto says no.

Here the TOR side of it. They should have some cap space for Barb next year.
https://www.capfriendly.com/armchair-gm/team/3292288

Leafs respectfully decline

Barbashev would be a no for us, Mikkola I like his style, you'll have to give us another piece

Doesn't matter we would prefer sanding as he will be RFA and cheaper.

TML fans: "OF COURSE teams are lining up to give up picks for guys like Jake Muzzin and Petr Mrazek!"

Also TML fans: "Ah, you're going to have to add to the package featuring the 60-point guy on a $2.25M deal if you want the RFA rights of our 4th-best defenseman"

Would you trade Sandin for Kerfoot @ 50% retained?

In a one for one? Probably not, but this isn't either. You're basically downgrading Sandin into Mikkola (still only 26 and probably coming in at around $1M next season) and getting Barbashev and upgrading a future 4th into a future 2nd. It also gives you the flexibility to move either Kerfoot or Nylander for more cap space to sign a goalie - you're probably all in at $3.5M tops for Barbashev & Mikkola, and can clear another $3.5M by just moving Kerfoot.

So you would do Sandin and a 4th for Kerfoot @50%, Mikkola, and a 2nd? Wild.

You've looked at TML's cap situation, right? They've got less than $8M in cap space next season and the only goalies they have signed are Mrazek, Kallgren, and Woll. They have 4 left-handed D signed for at least the next 2 years (Rielly, Muzzin, Brodie, Giordano). If you're asking if I'd sacrifice a young, left-handed D to put myself in a position where I can consider either upgrading Alex Kerfoot and being able to move his $3.5M without worrying, or think about moving Willie Nylander's $6.9M without sacrificing all of his production, I am 1000% going to think about it.

Glad you're not Leafs GM

First, there's less than no use for Beaulieu, who the Blues have routinely torched for years, or Sanford. Second, Toronto is almost certainly not doing that and the Blues probably don't want Sandin anyway. The second trade, I see no reason to pay so much to shed a player that was kiiiinda low-key badass the last several months. Nor do I see any reason to spend so much cap space for next season to get Allen. So not much thumbs upping, I'm afraid.