No cap in playoffs, only roster limitations.
I was talking about regular season, after deadline, where one can trade player rights for picks since no roster implications.

But not central to conversation here.

Huh?
The fastest route to his SPC is to sign with Wild.
Would sign 2-year ELC with Minnesota, and he burns one year in Iowa.

Telling Wild in March that he is not signing, means he wants to play elsewhere.

You can trade players in the regular season after the deadline. Any player. Not just rights or picks.
It just doesn't happen frequently because those players are then ineligible for the post season, so there's less of a market and demand.

If a team in LTIR has a player return during the regular season that puts them over cap, they would have no choice but to make such a trade, is the example I used.

The fastest route to an SPC is signing anywhere, not just here. It's to his benefit to let Guerin know that he won't sign here unless he can burn the first year. That way, if Guerin isn't willing to burn a year, we can still recoup some value and McBain can get traded somewhere that would burn the first year. If McBain decided to stay silent until after his season is done/after the deadline, then his options are to hope Guerin wants to burn the year or wait until August and sign a 2 year deal as a UFA. It's mutually beneficial for McBain to have communicated his interests to Guerin ahead of the deadline.

Have Wild said that they wouldn't sign him? His choices are

(1) Sign with Wild in March/April, 2-year ELC that begins in 2021-22 (even if he spends the balance of this year in Iowa)
(2) Become UFA, sign 2-year ELC that begins in 2022-23 with new team.
(3) Get traded in March/April, sign 2-year ELC that begins in 2021-22 with new team.

If his motivation was to get through ELC quickest, he would have chosen #1.
If his motivation was to choose where he plays, he would have picked #2.
If he wants to do both, and help the team that drafted him, he would chose #3.

(Also, even as UFA, he is still signing 2-year ELC)

So trade him to Vegas, then they trade someone else to make space?
I mean, sure, but it seems unlikely they would bump a veteran to add a NCAA newcomer.

I am saying the March 21st trade deadline means less to McBain.
He could have been traded March 22nd, signed with new team for 2021-22 season.

He informs team now (before his NCAA season is done) so that the Wild could get something in return for him.
Wild return the favor by allowing his agent ("advisors") to work on finding that trade.

I scratch your back, you scratch mine.

Nonono. I'm not talking about making a trade.
You said you can only trade draft picks and player rights at the deadline. You can in fact make any trade as normal.
The trade deadline is actually the deadline to register players for the post season, it's not a "trade" deadline at all.

Oh, yeah. You are talking generally. Yes.