I doubt Sens will take Karlsson at this point. Their top 4 don't need him, and the Sens will be paying Jake Sanderson next summer, so putting that much money into defence is not logical.
Sens need and want a goalie. No matter what, a starting goalie will be coming to the Sens this summer because that and having Chychrrun for the year should make them a competitive playoff team.
If and really if Debrincat is traded, it won't be the end of the world as Sens's top 6 is stacked for next year; adding in a top prospect winger in that position would be ok, or there are many LW available to fill that role like Killorn.

Why would San Jose be adding a high 2nd exactly? Terrible

This is an Eklund rumour. Which is basically the same validity as one of our made up trade proposals.

Not to mention, if the Senators are giving up Debrincat, they aren't giving up additional 1st round assets for Karlsson. Because of the nature of the 2021 draft where players were hugely under scouted, giving up their 1st next year would put them in a position where they haven't drafted a 1st round caliber prospect in four straight seasons. Not to mention, they would only have 2 second round picks in that same time span (since they aren't going to qualify Roger).

If Alfredsson is involved with the new management group, the idea of Ottawa acquiring Karlsson is not that farfetched. If the cost for Karlsson at 18 percent retained is a 1st, and two "1st round prospects" (which can mean anything, Boucher is technically a 1st round prospect), then Ottawa could make it work but it probably means they have to move Debrincat to get draft picks and then one of Zub, Chychrun, or Chabot to both open up cap for a goalie and recoup assets.

Chicago is going to need players to play with Bedard. A 3-way deal between San Jose, Ottawa, and Chicago might make more sense if it is:
-Debrincat/Zub/Forsberg (cap dump) to Chicago
-Karlsson (with retention from San Jose, maybe also from CHI) to Ottawa
-Chicago sends assets used to acquire Debrincat/Zub to San Jose for Karlsson going to Ottawa.

Although, Chabot makes more sense to move than Zub if we're talking about adding Karlsson to Ottawa, because Chabot and Karlsson would play a similar role. Although, I'm not going to open that can of worms. If Chabot was in the trade, it would probably be Joseph as the cap dump instead of Forsberg.