Admittedly, while I have the rump United States as the most likely contender for the Western Reserve, the issue is that New York probably won't go down without a fight and there's the whole matter of the Erie Triangle to deal with. I have a major feeling that it will be super ugly. Honestly, I could write a whole chapter or two about Ohio if I wanted to. Southern Ohio in particular should be a notable backdrop for major conflict with multiple states competing for it. I'm not so sure about Tennessee-Kentucky though since I'm not sure they'll full on unite with Virginia to keep them being drowned out by Richmond but the southern parts of Ohio, Illinois, and Indiana were largely settled by people from the upper south IOTL if I recall correctly. We'll see how this plays out.The Western Reserve could be sold on the condition it gets to be its own state. That would be a good way to placate the locals and assuage Connecticut's conscience. Even today, the Western Reserve is a bit different from the rest of Ohio. At the time, it would not have enough population, but legal niceties can always be stretched. The other states left in the Union could support statehood for such a small area as a way to dilute PA's power further in Congress.
As for another part of Ohio, the Southeast, the Ohio Company should have already purchased their lands and created Marietta. Since this land grant came from Congress, the Ohio Company, and by extension, its settlers, would have the motivation to uphold the Union as their legal rights to the land come from it. Southeastern Ohio could be another state added to the Union. This state's western border could be a problem.
With the collapse of the Union, Virginia could make a claim to the Virginia Military District, an area in the southwest of the state that Virginia kept, and Congress recognized, for land grants and as bounties to discharged soldiers. If Kentucky-Tennessee united with Virginia, that could make a new state in the proposed Union (and potentially drag slavery north of the Ohio).