Let go over these one by one:
Lnyx vs US armor: US armor is more than likely one to two generations behind the rest of the world, if not actually still on WW2 or Mexican intervention ideas.
US has been stated to have an M26 analogue about 10 years ago, IIRC. They have been stated to have developed the M3 90mm gun, so full M26 as of the early 50's (about on schedule). After Greece, they have seen how powerful the 88, which as we've last heard for sure, still is the Lynx's main gun. Additionally, even if they can't get a full gun, ammunition for the 88 and 128mm guns should be easy to obtain, giving you case capacity, rough chamber pressure, with known barrel length, which gives you a rough muzzle velocity and ergo penetration capacity when combined with the projectile mass, metallurgy, and design.
The US, if it's even 1/4 as competent OTL, should have a good idea of what German guns can do.
Second they have ALWAYS had excellent gun depression and strong turret faces. Which is exactly the type of fighting Korea will be.
Third they developed their own composite armor in house 10 years before anyone else.
We have zero indication that US tanks would be crap from a technical standpoint.
The Russians are definately on the German and Korean side, they most certainly do not want a resurgent China across the Amur river from them if not actually wanting a independent Manchuria in the Russian sphere of influence.
Britain and France are not going to be left out with Canada in play and France having a way to get back into Asia after having lost SE Asian colonies.
Lacks tubes? This is this time lines UK not OTL UK, THey still have an East of Suez mission.
UK if she wants to jump into the frey, but it sounds like she's just going to be an opportunist in this round (hence Jack and MI6). France may very well do the same thing. It's doubtful Germany would let her take colonies.
And the UK was short on tubes (especially modern ones) from the very beginning of the 20th century. Hell, before that. She entered the Boer War short of tubes (especially modern ones), ditto for WWI, same story for WWII.
Japan unindustrialized? No just not able to make next gen weapons, could make current gen easily, just have to have the plans to do it. You did not have the whole sale destruction of Japan like they did in OTL WW2.
And even before WWII Japan was unindistralized.
They lacked capacity for steel production, they lacked any strong automotive industry, about the only thing they did have enough of was ship yards (except against the USA who cranked out over 150 flat tops) and aircraft manufacturers, but still couldn't get enough engines for all the planes.
Remember this is Chiang's type of army from pre WW2. All the corruption and keeping good troops out of the hands of possible rivals here. All that surplus US equipment that they sold them also.
Literally all they need to do is be a human wall.