The lesser foes you speak of are terrorist groups in Syria and Afghanistan. This is basically a training exercise for something like the B-2 or F-22 ... it is not a validation of their combat capability. The only war that somewhat comes close is the NATO intervention in Yugoslavia, where I'm sure you can remember a thought to be undetectable F-117 was shot down by a unsophisticated Soviet S2A missile. The only two Chinese 4th gen fighters that have been available for export are the J-10 and the FC-1/JF-17. The former while advertised I don't think was ever pushed for export as much as the latter. Beyond a few placards, I don't think the Chinese aggressively tried to push the J-10, unlike the JF-17.
Regarding the Su-35, the reason why the Chinese bought them was simple ... for their TVC system (specifically its integration with FCS), to evaluate against the state of the art Russian technology (create an OpFor force), and to serve as a stop gap measure before the J-20. I am sure back door politics also played a role, since the purchase happened within a couple months of the S-400. If anything, we already know from Chinese sources that while the Su-35 is currently the most maneuverable aircraft in the PLAAF, its BVR capabilities far quite short of those of the J-10C and J-16.
Your point on air wars in Syria and Afghanistan are noted. The B-2 delivered strikes on Saddam's Iraq (for what it is worth).
F-117 was a 1st generation stealth aircraft. Was the F-117 ever evaluated by anyone outside USAF or USA ? Nope. The F-35 has been acquired by dozen modern air forces.
The Chinese purchase of 24 Su-35s is to reverse engineer technology. Pure and simple. On the Su-35 what makes you think Russians are going to sell state of the art BVR capability to China ? They put inferior BVR weaponry and radars on the Su-35 to China
Have the Chinese any success with JF-17 ? Not really - Nigeria and Myanmar are hardily professional air forces. The sale to the PAF is with Russian engines. The PAF has a limited budget.
You are claiming China's ability to make 5th generation fighters when there is no evidence they can make a complete 4th generation system for export and sell it