Why can't china be able to have a naval base on the Kuril islands which can allow it threaten Japan and have free access to the Pacific ocean
I don't think China would want it even if they could have it. It would kind of make China responsible for its security after as the closer, stronger power. Same with Vladivostok. Even though it used to be part of China, I think today it is much more useful in Russian hands. Since it forces Russia to care about the western Pacific and not just rely on China and get a free ride like during Korean war.
Let Russia keep Japan busy, and this time Russia can pay the blood cost unlike last time during Korean war when China paid it.. this time it can be China who funds and arms Russians to fight against Japan. But of course china will also probably be involved somehow, unless it's a direct fight between Russia and Japan over Kuril Islands or their disputed territories.
Russia also needs that port in Vladivostok since it is the only year round port. I mean if you look on a map, the land itself is not that useful for China. If anything it makes China more vulnerable if China was to have to defend the whole of the Russian far east Coast + Kuril islands. It would also put china very close to Alaska and USA. so this way, just like china has NK as a non US bloc(and even anti USA) buffer zone, it also has Russia for the same thing. It doesn't get any better than that. And as for Russia itself, China doesn't really need to worry about the Russia attacking or invading China from there.. they need to worry about Japan and China if anything since it China wanted it could probably take it anytime(nukes not withstanding). And if things really got serious and Russia needed Chinese help, China would be able to use it easily as needed. Maybe china will even get to take it back from Russia in the future just as Russia took it from China in the past, but like I said, it is a bit of a hot potato and having Russia have a permanent port and access to west Pacific to counter Japan and also be part of a china/nk/Russia bloc in that region is very much in chinas interests. If China controlled like Vladivostok and kuril islands Russia would probably be very happy today since it's one less thing to worry about or having to spend lots of money defending. lol.
As far as China is concerned, most of the invasions have come from that direction. Especially Japanese invasions. So Russia being there and also being in a territorial dispute is a great thing as far as china is concerned. Even more so these days since USA and West have completely lost the plot and pushed Russia 100% into chinas arms/sphere of influence. It would be much more dangerous for China if Russia was "neutral" and there was chance of Russia switching sides and working with USA or West or Japan against Chinese interests but after Ukraine and related sanctions, that doesn't seem likely at all.
As for Japan, China has medium/long range missiles and subs and big navy already and also air force which is more than enough to threaten Japan from the mainland. An extra base on Kuril island won't mean much, and if anything it will be vulnerable by itself if China also doesn't have Vladivostok and all the Russian far east to supply and co ordinate with it. Which like i said would be very costly and open up another front for china to have to worry about.
Imo it would be overreaching for China to have just one forward base in Kuril islands and not have the whole Russian far east including Vladivostok. Chinas doctrine revolves around active defense of whatever they call it, and based around A2-AD umbrella. I think a lone base on kuril would fall outside such umbrella. Just see usa. They had B2 bombers and other jets like F15 in Japan, but they have all pulled them back to Guam or us mainland itself. Why? Because it's just sitting duck and would be too easy to be destroyed by Chinese missiles. I mean what are they even going to bomb anyway? Chinese mainland? Yeah, if they want USA mainland to also eat ICBM or SLBM. If Russia is already co operating with China, China can just fly to Vladivostok or whatever and attack Japan from there. So no need for permanent base. It would also go against China's "non interference" alliance foreign policy and the small benefits is not worth giving up that moral high ground after keeping it so long..
Lastly, China is not an imperialist power. It just wants to defend itself and it's interests. It doesn't need or want to do global domination or attack Japan etc. that will only lead to increased tensions and arms race, maybe Japan even goes nuclear after. Such arms race is not smart and does not benefit china really.
China is fine with status quo since it is winning either way economically and technologically. I mean, can USA or Japan or whatever blockade Chinese even though they currently have the "first island chain"? Can they block chinese import/exports from strait of Malacca and cripple Chinas development? Can USA attack or bomb Chinese mainland even though they have bases in SK, Japan, Oh and have B2s and B21s and B52s etc? They can try and probably can do it, but in reality they can't because japan, sk, ph, will cease to exist and USA mainland will also get destroyed in such a war.. So it's just a waste of money and mainly for their own egos. Nothing for China to really be worried about, and certainly nothing for China to need to create such offensive forward bases which will just make things more tense and could just lead to arms race and possible accidents and escalation which will only have one ending. Everyone dead. That is a fools game, let USA play that dead end game.
Russias current geography + situation means it has no choice but to act as a buffer state for China against Japan/USA. Just like NK is doing a good job as a buffer state against USA. I guess China is in the big leagues now when it has Russia as a proxy/buffer state. It used to be the other way around just 70 years ago. Haha. But yeah. China will take care of Korea, Russia can take care of Japan and Kuril islands and that upper Russian far east coastal areas.