Fearless Leader
Donor
The seniority lists called. They want to laugh at your idea of having Perry go there. Or a good replacement to Dearborn, for that matter.
Which is a shame, because Perry is the kind of man you want. Same with Harrison.
But both are way too junior to for an Important Project of this sort, particularly before the ancient relics like Dearborn are proven to be worthless.
Easy enough to get rid of him personally if his health is that bad, however. That should make things at least less bad.
Perry's not as far fetched as it initially may seem as service on the Lakes was seen as a low prestige, backwater posting. Most officers (Chauncey included) wanted to fight the Royal Navy on the Atlantic, not piddle around in Lake Ontario with tiny boats and such.
As for a feasible replacement for Dearborn, both Harrison and John Armstrong Jr. are well within the range of possibility. I have a hard time seeing Harrison, the Hero of Tippacanoe being regarded as too junior for such an operation. IIRC in OTL his name was considered to lead the expedition up to Detroit. Armstrong's definitely the lesser of the two, but he had a good grasp of the overall strategic situation as well as half decent command characteristics. Had he not become Secretary of War in 1813 I think he would've made a half-decent commander. As he was a major force in determining US strategy, I could easily see him volunteering himself to lead the attack on Kingston in the absence of Dearborn.
I have a hard time imagining him saying "Oh well, I guess we should just abandon Upper Canada." - and even if he says it, London accepting it instead of using it as a reason to find someone who will retake it from the pesky Americans.
Makes a big change in the war, and Prevost may well do what you said - but the response to those changes and his actions may not lead to more American success.
Looking at how things work out in North America, not final treaties, as what influenced those is unfamiliar to me. Most of what I know is on the naval side of things.
The thing is, if you look at the allocation of British troops in North America, Upper Canada was effectively written off from day 1. For the entire war the bulk of the British forces in North America were sitting in Quebec and the Martimes with only roughly 30% being stationed in Upper Canada. Furthermore Prevost proved to be incredibly reluctant in providing reinforcements hinting to me that he saw Upper Canada as a "lost cause". Should Kingston Fall, I think his immediate tactical response would be to fortify Quebec and hope to hold off the Americans until reinforcements can arrive. Considering that there is a much larger conflict going on in Europe at the time (War of the 6th Coalition) I don't see London having a big problem with this. Add into this a more fruitful armistice and earlier negotiations and the War of 1812 becomes just that a short victorious war for the Americans with little actual territory changing sides. Sure Britain's pride is a bit wounded, but now she's free to concentrate on Napoleon...