I would make that deal for sure.
I get that Larsson is playing pretty well and fills a niche as the only real shutdown option on the right side, whereas Barrie/Bouchard are all offense and Bear is having some real hiccups. But that's a far easier position to fill than top-6 forward.
I don't know what's happening in Buffalo, but Hall isn't playing as poorly as it would seem. On that tire fire of a team Hall is somehow positive in CF%, and xGF%, but his PDO is 960, he's only shooting 2.3%, and the team can't get a save or finish a chance with him out there. He is generating genuine offense and nothing is cashing. I'd take a roll on those dice.
Now, you could argue that what the Oilers really need are some finishers. That McDavid and Draisaitl can create enough chances and if Puljujarvi, Chiasson, etc. could bang more home then scoring wouldn't be a question. Hall's primary skillset is playmaker, and that would be lost a little on a team already featuring McDavid and Draisaitl. He wouldn't help their already ludicrous PP. It's a bit overkill. Whereas Larsson's shutdown skills fit the roster better.
You could also argue that the bottom-6 is where the scoring is really needed. Get a 3rd line that can outscore teams and we could be laughing. That would make 3C a potentially more important spot to upgrade (come on Derek Ryan for peanuts), but I'd think Hall for Larsson doesn't preclude you from trying to get a 3C on a near minimum deal. It just means you have to find a Larsson replacement, or maybe you tilt further towards all-offense as a strategy thinking that you can absolutely run and gun with a top-6 of McDavid, Draisaitl, Hall, Nuge, Yamamoto and Puljujarvi.
So if you did make the deal you'd have to find a Larsson replacement. There are some RHD shutdown types on the trade bait board. David Savard is #1, but what is that costing you? Josh Manson would cost an arm and a leg given term and complicates expansion protection. Ekholm could play the right side but is ludicrously expensive and maybe not even available. They also don't have the cap space for any of these options.
You also wonder about the mix in the room. I suspect Larsson is well-liked/respected as a dirty work type of guy. Would major surgery like this upset the apple cart too much?
But in a vacuum I do the deal, because it's a talent upgrade. And if I'm getting a talent upgrade then it becomes more realistic to take a run this season, so adding even more becomes worth considering.
It's a fun thought experiment because it's something that isn't realistic, and we all know we're headed for a quiet deadline.