At least one Briton is going to be fighting on the side of Italian independence. I'll get to him soon.
What's the issue?
If it's just that the border I drew is kind of jagged, remember that (like the one in the Treaty of 1818 IOTL) it was created by people who were trying to sum it up in an unambiguous written description, and who only knew the land from the charts and descriptions of a bare handful of explorers. Under the circumstances, it could have looked a lot worse. IOTL we ended up with strange possessions like the Northwest Angle and Point Roberts.
From an ambiguity point of few, given the paucity of information... It seems they would have just picked a line of latitute don`t you think...46th in this case. Or more to stretch it...the line of latitude running through the headwaters of the Red to the Columbia River or the Pacific or at least the Stoney Mountains (Rockies)
I digress though.. The NW would not even be a consideration...and Russia would not even care, given that the US has no way to project influence there. The US only received a concrete claim in the region after Adams-Onis, hence border considerations would likely only be centered largely with the area to the East of the Rockies known. until after the border is settled with Spain. Britain will simply dismiss the US pretensions. Hence until they sign their treaty with Spain, which I presume contains the same provisions with repect to there claims in the NW that Adams- Onis did. Negotiations on the NW will not even enter the picture.
Of course if there were a prid pro quo....say extending that border eastward to Lake Michigan., or giving the British the entirety of the Selkirk grant at least...then maybe, the British might be in a talking mood. Even then the US is severlely disadvantaged as they have no presence in any of the lands in question to begin with.