If a high intensity war between the US and China breaks out in 2028, it will go nuclear and what Canada could do is meaningless.
Extremely Unlikely. This isn't a 1960's cold war stand off. But its interesting you say that, maybe that feeds into Canada's position. That if the US is defeated/stalemated then from the Canadian point of view, there is no point in trying. Canada needs to ask more questions of itself, including what if the US is defeated, or if the US abandons them.
China has a different policy with nuclear weapons, they aren't are major part of why it is a threat. The truly game changing thing from before, is China doesn't need nuclear weapons to win against America. China makes more ships, more planes, more missiles, more technology than the US. China's economy is as big as the US. The US China relationship is very, very different from the US-Soviet Russia one.
China doesn't really want to wipe the US off the map, they would make excellent consumers for Chinese goods, it wants the US wealth, and it wants to get it by simply beating the Americans at their own capitalistic game and make them secondary to them. The US isn't some ideological devil that can't exist, it is what they intend to replicate and turn themselves into. America needs to remain materially intact enough to prove that China is better. At least in my opinion, analysing and reducing complex nation states and their aims is difficult and not my forte.
But if Canada thinks the US is erratic and chaotic now, then looking into a future where China wins a conventional war, but with heavy losses on both sides, one that China quickly rebuilds and the US can't. China takes Taiwan, pushes Japan back, pushes the US Navy to Hawaii, neutralises Guam, controls global shipping. US becomes 2nd largest economy in all metrics. I would think the US would have its own internal crisis when that happens, it will become very chaotic and nationalistic, and may want to pick fights it can win.
Or US China competition just becomes extremely high stake as the two nations cold war try to out innovate each other in a brutal way, that involve various minor hot wars. Testing each others barriers and borders and defences.
One of the key things will be Canada in that situation.
China circumnavigated Australia in a rather limp attempt at perhaps intimidation (but still with a very capable cruiser, disrupting air travel, probing defences). I could easily see China deploying a more substantial force to push freedom of navigation through northern Canada, flying through Canadian airspace etc.
In that case US fighters could be shooting down Chinese drones over Canadian cities/urban areas on a regular basis.
In that case having an operational fleet of 100-200 older F-18 would be more useful, than a dozen F-35s. Canada would need to be able to fly continuous air patrol missions more like other countries like Japan, Korea etc. The Canadian air force would have to have much more mass. More squadrons based remotely etc. Many countries have large fleets of older planes like F-5/old block F-16, etc for just this mission. Taking on aircraft of a type already in operation with the air force would be much quicker and doable than trying to get deliveries of a new aircraft to bring in 50+< 3-4 years.
Canada could conscript/incentive former F-18 pilots/maintainers into service. Draw ex-US service men and women over. Protecting Canada may make more sense to them than being conscripted into a high intensity attrition war with China over Taiwan.
The F-18's I am talking about are now currently used as home guard units for the US. The aircraft would then perform a similar mission for Canada, not be deployed to Europe or Asia, but fighting over Canadian territory has Canada experiences a Ukraine style air threat.