A. In this case, however, the US is not armed to full capacity, and its army is woefully low in numbers as well as equipment. Major industrial regions being close to Canada doesn't help much either. Those two factors will very much help the Canadian war effort. A preemptive strike will mean that Canada would probably focus on destroying these areas, not holding them, and this will severely damage the US capability to produce weapons and things that will help them in the long run.
B. The RN will no doubt blockade the US, and the naval bombardment of those ports will keep the advantage on Britain's side.
C. Fair enough, but capturing Hawaii would indeed be an important asset, and there is a reason why Japan attacked it OTL. If a concentrated attack can be made against a resting Pacific Fleet, it can push the odds heavily into the Commonwealth's favour.
D. Mexico would not be a curbstomp, not if there are better supplied Canadian soldiers attacking major US centres of industry. They will indeed pose a viable threat in this scenario.
67th Tigers? Is that you?
More seriously though:
A: The same applies to Canada, as nearly all of Canada's major industrial centers are within 100 miles of the US border. If the Canadians were to completely remove the gloves and use chemical warfare upon Detroit for instance, then you'll do a bit of damage to the US Industry in the Great lakes for a while. Of course, that only means the US will respond in kind...
Furthermore, the US is Canada's biggest trading partner and without access to US raw materials Canada's economy goes into the tank, and this is without US retaliatory bombing action.
The US also has the largest rail network in the world and can quickly shift its centers of Industry. How do you expect the Canadians to hit places like Los Angeles, St Louis, San Francisco, or anywhere that isn't 50 miles from the Great Lakes region? Does Canada have any heavy bombers that are even capable of inflicting that kind of damage?
Again, a pre-emptive Anglo-Canadian strike that does anything substantial to US Industry would only guarantee that the US will not negotiate for a status-quo antebellum and that they'd want to fight for the long haul.
One other thing. How many troops are the British willing to commit to Canada? Remember that they have colonial commitments everywhere. They can't afford to remove troops from India, or it'll immediately sense an opportunity in rise up in revolt. In WWII, even with the Japanese on the doorstep in Burma, India was actively protesting against British colonial authorities. Also, this risks Soviet encroachment into Central Asia. Do you also divert troops away from Europe when Hitler and Mussolini are actively re-arming for a showdown?
B: So the US navy is just chopped liver? The US has at least naval parity with the British when fighting close to its own waters, and furthermore the British have to deal with land based US fighters. Seriously, have you seen a map of the US Eastern Seaboard, and the number of effective ports it has? You don't think that each of these ports will be brimming with naval mines and coastal battery guns?
Furthermore, how long will it take for the British to mobilize a fleet that is scattered all over the world to be in a position to blockade the port? What prevents the US fleet from transferring ships via the Panama Canal? How do the British prevent the US from sailing ships from the Gulf of Mexico and neutralizing the few port facilities, rather, the ones that are actually capable of repairing and refueling battleships and carriers, from US naval action? How do the British put a stop of naval production coming out of the Pacific coast? Britiain will also need to maintain huge merchant convoys to continously supply Canada, which will come under US submarine action sooner or later. As mentioned before, once the US can block up the St. Lawrence, then there's no more way that the British can send significant reinforcements to Canada.
C: As previous posters mentioned, the Pearl Harbor attack was hardly the decisive blow that the Japanese intended. And who is to say that the British will be willing to get the Japanese involved anyhow? Their attention will be focused on China. Furthermore, what message does this send to Australia in New Zealand by fostering Japanese expansion in the Pacific? Again, if you have a pre-emptive attack on the Pearl Harbor fleet you'd have the outrage that only guarantee's US resolve to fight it out to the finish.
D: That worked very well for Pancho Villa didn't it? Besides, US relations with Mexico were actually pretty good during the 1930s because of the Bracero program, which allowed Mexican immigrant workers to make money in the US. Why would the Mexicans give that up in exchange for vague British promises for territory in the Southwest US. The minute that a Mexican president even thinks about war with the US, you can be sure that a coup will follow. It also doesn't help that the US can completely blockade Mexico whenever it feels like it.
We can switch the situation and point out that the US, with its large American-Irish population, might also try and send clandestine agents to Ireland to cause trouble for the British. If the Irish stage some kind of revolt, it'll be a running sore that the British are obligated to deal with, and assuming that the US can maintain naval parity in the mid Atlantic, they can insert a few Irish-American agents into the Emerald Isle to cause some trouble.
As for supplies, the Canadians have their initial stock, but sooner or later they'll need to be re-supplied.