They will buy, what they will sell them, and for secretly training against the S-400, I doubt it, again any information gleaned about Chinese air defenses, I doubt it.
What? That makes no sense...why would any country buy what doesn't fulfill their needs? If for whatever reason J15D doesn't fulfill Pak's needs...Pak WILL NOT buy it PERIOD.
Ur doubts are unfounded based on no evidence. Pak with its limited budget(that puts huge constraints on R&D) has been able to "develop" certain key technologies that only "rich"(big economies) countries usually have the means to develop.
- How much do u think it costs to build ballistic missiles...add to it the cost of developing both liquid and solid propellant based missiles(Ghauri and Shaheen series). The latest addition to that still "under development" is MIRV tech.
- How much does it cost to develop cruise missiles from scratch? Pak has ALCM(Raad) and several variants of Babur, which includes submarine launched version. Then there's also Zarb and Harba. Developing cruise missiles and all those variants requires deep pockets or lots of help from those that already have that tech.
- No doubt Pak's nuclear weapons program along with the tech to enrich weapon's grade fissile material also got help from China.
The above are just some of the key technologies that China has helped Pak with. No country helps another with nuclear and missile tech. Of course neither of them would officially accept to this bcuz of the repercussions...but does it not make u wonder how Pak almost always runs SUCCESSFUL tests whether it's to test the SLCM version of Babur or MIRV capable Ababeel or anything else? Pak doesn't have that oil wealth...nor does it have a huge amount of scientists/engineers/etc nor a developed industrial base like the western countries. If Pak was to do all this alone without Chinese help...Pak would've ended up with lots of Tejas, Rustom, Arjun like examples.
If China can help Pak with such strategically important stuff(like nukes and missile tech) decades back...then a J15D that can counter S400 shouldn't be a big issue. Moreover there has never been an issue from Pak's side...of any info being leaked to US or any other country for that matter.
Based on the above I'm inclined to believe that China will not hold back on providing Pak with the means to counter the S400. S400 is among one of the best air defense systems around rn. If it is deployed in India in key positions(near the border) against Pak...it can pose a great risk to PAF...
...a compromised PAF(operating in a dangerous SAM protected environment) against an IAF that's massive in numbers(with added Rafales)...would give a huge advantage to Indian military...something that Pak may not be able to overturn or recover from. China wouldn't want that...China would want Pak to have the capability to keep India busy and at bay.