Well, a clathrate release only hangs around in the atmosphere for a hundred years or so, since methane's got an atmospheric residence time of something like 15 years IIRC. So that'd be a brief heat boom -- this is the supposed PETM trigger, and that was an incredibly brief blip, less than 100,000 years.

The volcanic release would be more significant, but I'm still not sure it'd be on a Siberian Traps scale. The last few times the Yellowstone hotspot has gone off, it's been well below 5000 cubic km -- enough to cause a volcanic winter and make some large herbivores briefly unhappy, but not enough to register as a blip on the extinction scale. Furthermore it's rhyolitic IIRC so you'd just get a ton of ash, though well enough to make it locally uninhabitable for a while. The Siberian Traps were something on the order of a million cubic km, and basaltic so they all stayed local.

The "nuclear winter" effects of the Chicxulub impact didn't make the angiosperms go extinct -- they did just fine, as did most of the bugs living in them, in fact. The part of North America that got the ashfall would be hosed as far as forests go, that is, but the forests of SE Asia and the Amazon should be alright, and the tropical Australian forests should spread. You've drastically reduced the amount of land in the 30° desert zone, and increased that in the tropical-forest zone. Plus there's a ton of new drowned shelf in the Baffin-Hudson-Arctic ocean area, which should stimulate the ocean ecosystems.

If you're putting yourself 100,000 years after the cataclysm, I think you should be past the clathrate warming, acidification, and volcanic-winter nastiness -- at least if we take the PETM as an example. Or PETM + Long Valley Caldera eruption, for that matter. You'd have to figure out what went extinct, though, and what's taking its place. But I doubt you'd be getting into P-Tr, "everything but the stromatolites is gone" type territory.

Regarding my comment about the thermohaline circulation (I love that image), you're nuking the very cold meltwater that sinks in the North Atlantic with this as well as breaking the circum-Antarctic current loop...