There certainly are factions in both countries who have major trust problems with each other but there are also factions in both countries at the very top who see the major strategic benefit, if not need, for that to change and the two to co-operate to survive/thrive in the face of containment and/or in case of hot wars with third parties which may well not be of China's or Russia's choosing.
Supposedly the Kremlin intervened to make this deal happen the way it did with planes rather than components, it's safe to assume Zhongnanhai approved of the deal from the other side. That indicates a push from factions in both leaderships to enhance defence relations especially if an aspect of it was either China overpaying for past offenses or Russia willing to do technology transfer, or a bit of both. Even if so it's merely a tentative step, so time will tell.
If we get into politics it will not end, the only thing i can suggest you is read about the opinion many Russians have about Siberia and the Chinese immigration into it.
No need to go further.
About why they bought Su-35 you have to see two important points.
Stealth is not invisibility to radars, it is only lower reflectivity, if the radar is weak, the detection range is just a few km.
To put it in context, a MiG-23 will detect F-22 probably at 10-20 km from it, but a Su-35 might be in the range of 100-70km of Range.
However the Su-35 has the ability of group scanning, via data link, MiG-31s can sweep areas and overlap their individual radar ranges and via data link they can survey 600km.
So a group of Su-35 can patrol an area and close the gaps of their individual radars plus they have data link so their IRST system can share information via data link.
Add AWACS, radar ground stations, etc etc.
So for China like any other nation 4th generation fighters are not going to be substitute with 5th generation fighters for a long time.
J-11s and Su-35 will be at least flying for another 10-20 years, why? well simple radars are advancing too.
I can assure you a fighter like Rafale or Su-35 will beat F-35 in WVR, thus if you detect a 5th generation; a 4.5 generation can beat it, it is similar to the Vietnam war era when the less maneuverable american fighters were defeated by older MiG-17s and MiG-21s and that can include fighters like F-111.
The reason is stealth imposes so much restriction into the aerodynamics and weight that the fighters need to remain undetected to fight, otherwise they have no advantage at all.