The Homare was a troubled engine and wasn't able to generate near its spec in practice until 1944-45, but still only producing around 1800hp.
Nakajima Homare - Wikipedia, the free encyclopedia
The 1200~1600 HP class engines would be the ones to realistically look at for the early.mid war period. Namely the Mitsubishi Kinsei, the larger Kasei, and Nakajima's Ha41/Ha109. The only reason engines really came up was in regards to the Ki-60/61 and whether they'd use an alternate inline design or resort to an earlier counterpart to the Ki-100. (the Kasei would have delivered similar power much earlier than the Ha-112 the Ki-100 used, but was bulkier and heavier. A lighter engine in the 1200-1300 hp range should have been possible earlier (at least around the time the Ki-44 came online). Either way these sorts of developments aren't really going to be relevant until around 1942.
With stronger opposition from faster, more heavily armed aircraft, some developments might have been pushed sooner, maybe with earlier engines but still some gains from greater emphasis on armor, self sealing, and speed over maneuverability. If the Ki-27 was taken down in droves by significantly less maneuverable aircraft in 1939/1940, the Ki-43 might have not existed as it did in 1941 (armament included), and the A6M might have seen design modifications earlier.
That or the Ki-43 might have been passed over in favor of some (hypothetical) competing design, say the 1940 counterpart to the Ki-28 that lost to the significantly slower Ki-27.
Also without a war with the Soviets there is not going to be a Panther design, rather a VK3001.
Given the timing, even STUG IIIs wouldn't be available initially, regardless of the ability to actually get them to the front lines. Panzer IIIs would be the only thing available, and even those would have been scarce if production hadn't ramped up (and ramping up production to support China is a bit different than planning to invade France).
That said, with the lighter Japanese tanks as the main opposition, the Panzer III seems like it would fit fairly well even as a tank destroyer. It might actually be too heavy for some of the terrain being dealt with in the region, and with the older Panzer II and Panzer 35 available in greater numbers (and easier to transport to the front lines), those might have been the more significant tanks of the early war. (the Panzer I seems less useful, but might have been pressed into service as well)
Doubtful...The area in and around Indochina had a large French colony presence and they would want to avoid involvement. This would include Cambodia, Laos and Vietnam. Burma and several other areas were British. On the otherhand, Thailand (Siam) was aligned with Imperial Japan.
In otherwords, allowing German access through their colonial territory would be taken as complicity by the Japanese and upset the "neutrality". If Russia allowed access to the far east, the "difficult peace" between Japan and Russia would be upset.
So Germany would really be having a hard time in getting any substantial assets into the China theater without upsetting the applecart.
How different would that be than support (or merchant ships) originating from other officially neutral countries during early WWII? (particularly all the countries American firms were selling weapons to) Or various countries supplying Finland with military equipment during the Winter War. Hell, I'm not even talking about Britain/France/etc SELLING weapons to China, but allowing passage through their territory ... or at very least for allowing certain material transports to pass through, including any return payment from China.
More so if there was also a passage available though Russia, meaning the indochina route would be auxiliary.
It seems like getting troops clear passage would be the bigger issue. Though some political finagling with merchant ships and trade routes supplying military equipment being /sold/ to China along with /volunteer/ personnel taking whatever passage they could arrange, more options might open up.
Easy, they are shipped in on cargo submarines.
Lots and lots of cargo submarines.
And yes ... in jest or not, some supplies could be sent by submarine ... more so if dedicated transport submarines were emphasized ... and if they were allowed to stop at neutral ports as 'merchant ships.'
But that's of course only remotely realistically useful if merchant shipping to China was totally impossible to manage without unsustainable losses. (and either way, you need access to secure ports on the Chinese coast to unload the cargo and vector it to where it's needed)
Unfettered no, which is why I think they will only get a reinforced air corps into China. The Zero was significantly slow, so as long as they don't dog fight and learn as well as the Flying Tigers, which shouldn't be hard given that the LW was far more experienced at this point, they would do great.
Not to mention the MGFFs of the 109E would shred Zeros, Ki-43s, and Ki-27s even faster than the .50s on the P-40B/C. (and significantly easier than the Hurricane, Spitfire, and some of the French aircraft the 109 historically faced)
That said, the 109 probably wouldn't have been able to absorb the damage the P-40 could, and range/endurance was substantially less, as was maneuverability at high speeds. (albeit still considerably better than the japanese fighters -along with higher G limits and maximum allowable dive speeds)
The Fw190 would just dominate anything the Japanese produced. Later when the fighter-bomber variant shows up it can outrun anything the Japanese had until 1944-45.
With conflicts in Europe foregone or delayed indefinitely (along with political changes within Germany), development of the Fw 190 may have changed as well, on top of the Fw 187.
With the relative performance of Japanese aircraft and requirements for that theater of operations, aside from the Fw 187 being reconsidered as a single-seat long-range fighter (and also one able to use up remaining stock of Jumo 210 engines and still provide compelling performance), you also have potentially greater emphasis on extending the Fw 190's range, or at least developing long range derivatives. Making room for more fuel and/or possibly even replacing the BMW 801 with a DB 601E (with ease of flying, range, armament and cockpit visibility all being advantages over the 109F) should have been realistic options, and even if eliminating the wing root or outer wing guns in favor of more fuel tankage, you'd still have some margin for a heavier armament than the 109. (even before the MG 151 was available, you could eliminate the inner wing guns in favor of fuel tanks and -in the case of the 601E, add a 3rd MG-FF/M in the nose, to be replaced by 151 or 151/20 later -or again, use the outer wings for fuel and focus on the centerline armament)
A DB-601E powered Fw-190A derivative might not have been as fast as the 109F (and almost certainly would climb worse), but it should have comprehensively outperformed the P-40B/C/E and even F while having more firepower and similar range. (probably similarly easy to fly, definitely easier than the 109, and likely easier than the 801 powered 190 due to lower weight/wing loading and torque)
Given the same situation would exist with existing Japanese fighters prior to the Zero, they'd probably keep the same tactics learned in 1939-40 and apply them to the Zero. As it was the Soviet fighters in Spain had the same advantage and the Bf109 was able to dominate anyway.
The Jumo 210 powered 109 variants employed there were significantly lighter than the 109E and were much slower. That said, the situation with the Bf 109E compared to the Hurricane Mk.I should at least be relevant as well, at least in terms of turn ability and roll performance being compromised at high speeds. (though not quite the same structural issues as the Japanese fighters, aside from very early Hurricane I's having wing fabric deformation issues, they probably handled high speed dives better than these japanese fighters, and certainly took more structural damage) The 109E would similarly have better dive acceleration than any of those, hurricane included. (better level acceleration and climb compared to some of them as well)