I am not saying the Germans should have magicly had hundreds of Zero's in 1940.
What I am trying to say is that the Japanese designers made certain choices to get the long range. Some of these choices (but perhaps not all) would have been available to any other fighter design team in any other country. However would some of those choices been acceptable to the generals/air ministries buying the planes?
Typically that was the major problem preventing better aircraft from being produced, more than the design limitations. Procurement policy took different directions in different nations and each had their strength and weaknesses.
Could the Americans, British, Germans or French and Italians have built a 1000 mile range fighter in 1938-40 given their available engines, their structural strength requirements, desired dive speeds? Once these nations started fitting some sort of protected tanks and some pilot armor performance tended to drop in 1940.
Performance dropped, but engine power steadily increased and it tended to equal out over time more or less, as in the case of the Mk II Spitfire. I think it is clear that in terms of design and production capability, it was possible to do so. But the decision didn't go in that direction right away. Basically of our six major aircraft producing powers: Russia, Germany, Italy, Britain, Japan and the US, The European powers focused more on interceptors or short range / frontal aviation types. It was the latter two which seemed to recognize the need for a long range fighter earliest which makes sense given geography. The Japanese initially focused on a long range escort fighter because of their experiences in China. The US foresaw a need to patrol long distances, envisioning the protection of their shorelines and places like the Panama canal. But once they got into it the long distances in the Pacific proved that need, and the longer (medium) ranged fighters they already had were helpful in the Med and elsewhere as well as for Allies in Lend Lease.
For the British the MK II Spitfire with the Merlin XII showed little or no performance increase over the early MK I Spitfire due to the increased weight and drag of the now standard protection and, increased ammo (not much) and extra electronics (the IFF). There are performance figures for early Spitfires with a fixed fuel tank under one wing. Changing to internal linkage (which there may have been room for) would restore most (all?) of the lost speed but the same can not be said of the loss of climb rate. ANd climb rate is an indicator of the ability to sustain speed in turns. a poor climbing plane winds up descending fairly soon after starting a turning contest. A good climbing plane could very well wind up having to descend but not quite as soon and would not have to descend at quite the same rate while doing the same turn.
That is a complex issue worthy of a deeper discussion...
Without lighting the structure and making it weaker what avenue/s did some of these countries have for making a ling range fighter?
Well ultimately it boiled down to threading the needle. Use lighter materials, design intelligently. Find ways to save weight, improve efficiency, reduce drag. The A6M was actually a stronger airframe than most people give it credit for. It was a carrier aircraft after all, and had a once-piece construction method rather than having the wings bolted on separately. It was very streamlined as well. The designers of the P-51 like Ed Schmüd as we know found all kinds of ways to reduce drag and improve speed and range.
Personally I think the design and engineering challenges could be met by most of the major nations, the bigger issue was the purchasing commissions, corporate bureaucracy, air force leaders and so forth having sufficient vision and prescience to know what they wanted and what was really going to be needed.
As for the Beaufighter, a lot of them had 2 speed engines. Not sure on some of the real early ones with Hercules engines but the MK II with Merlin XX engines certainly did. Quite of number of the middling Beaufighters did, Some of the later ones had the two speed supercharge locked in low gear for low altitude missions. I would also note that a Hercules XVI engine on 100 octane could make about 300hp more at most altitudes than a Hercules III engine on 87 octane could so be careful as to what combat examples you pick to show that the Beaufighter would have made some sort of day fighter for air to air combat. It was a very large airplane for a "fighter"
I am aware of the 2 speed engines etc. with the later model Beaus, and I see the limitation in the day fighter role, but if you look at the Operational history, Beaufighters, especially late model ones, did pretty well in a few scrapes. Pacific Victory Roll seems to be down right now for some reason but if you have a look at that some time it shows a fairly impressive tally. In the MTO they seem to have fallen prey to Bf 109s a bit more when they encountered them, the difference is probably speed - Beaufighter was close to the same speed as a Zero down low but much slower than a 109F or G.
I would concede though for a true escort fighter type you would probably need a bit smaller and lighter airframe. The Beaufighter had 10 guns, two crew and a 1,600 mile range. I think as I mentioned before, 4 guns, a 1,000 mile range, and the higher altitude capable engines might have been an interesting experiment. The zippy Whirlwind had a wingspan of 45 ft, whereas the Beaufighter was 57 ft. Maybe the modified Beau could have a thinner wing around 52 ft like the P-38 Lightning, and perhaps inline engines like the Merlin XX on the Mark II. If they could get the weight down to ~15,000 lbs you might have something there. How much do 2 x 20mm cannons and 4 x .303 machine guns weigh with ammo?
Of course all rank speculation there, for which I apologize. I do have a weakness for that kind of thing.