I am not sure whether I can technically answer your question or not, but here is mine.
If you take a glimpse at history, You will understand that, the EU is a big continent with lots of small countries (~30) and had seen some extreme wars. Post WW2, they all united into one unit (EU) to ensure they all stay together for the next war, if any.
Soviets had plans to conquer the EU nations and other countries. They were successful to some extent. EU nations (With support from the US and other allies) were successful in fending off the soviets from their land. US was strongly against Soviets and they don't believe them no matter what. (Cuban missile crisis, etc)
Post WW2, The Americans and EU's formed many treaties and bodies to ensure they work together and to stand against their enemies Together. As a part of this deal, Americans, EU nations provided the much needed financial aid and gave free access to their allies, so they can prosper and be potential allies to them when needed. As a part of this deal, the US provided their niche weaponry to combat for its (considered) allies like Turkey and others on (highest priority).
Some countries gobbled the (generous?) resources provided by their allies (US and EU) and when in distress, they started to slide with their opponents(For more resources? like a parasite?). (Greece threating to leave EU, Turkey threating to release migrants into EU). Providing (Financial/other) aid was never an issue with the Americans/EU nations. Forgiving 100 Billion debt to Greece wasn't an issue for EU. But, they want to enforce responsibility so they live within their means. They did everything to ensure the bad blood doesn't spread. But some countries started to take leverage of the generosity of others and started blackmailing with threats.
F35 is a niche war weapon from US. F35 is like a sniper, which is very good at doing one job (considering its cost). Other warfighters from the US like F22's or F16 are more like Green berets. They can survive in any environment and can combat in any situation. In a war, you can let your green berets anywhere. They are capable to fight in any situation. But revealing your sniper's position could change the game completely.
Now, why would the US allow countries like turkey to have access to its F35's (Snipers)? Once your not so ally is aware of your weaponry weaknesses/limitations, it could just jeopardize the whole strategy.
If US loses a dozen F16's or F22's, no one bothers about them. But If US loses 3 F35's, everyone will lose their mind. It could change the perspective of the US (citizens) and its allies completely. Strength is a perception. If it fades, you aren't commanding anymore. To ensure no such thing happens, you wanna conceal your weaponry. You shouldn't reveal any weaknesses. So, the US doesn't sell its F35's to any country which procures weaponry from Russia (and China?). Access to F35's was given only to (considered) essential needed allies. (Turkey, Singapore, some EU nations, UK, etc). Now, some of them started to misuse the privilege. How do you feel?
If you look from a shop's perspective, A shop selling only "Brand A" products will attract only "Brand A" fans. A shop selling "B Brand" products will attract only fans of "B Brand". If a third shop starts selling products of Both brands A & B, who is going to lose? What happens in war? The one who procured niche weaponry from both the US and Russia will be stronger than US or Russia. Why would the US force itself into such a situation?
Also, if Russia is the new vendor, they could demand a sneak-peek into F35's. Anyways turkey has jeopardized its relationship with the US and they need to mend fences with Russia, so yes. they will allow. Where will it lead?
Can F35's Fend off missiles from S400? I think, Yes. Will the US ever confirm that? NO. If they confirm, S-500 development will begin immediately. Why would they do that? Forget about the data of both S400, F35's etc. They were never in a war. (Except for some surgical strikes, which doesn't reveal anything)
In conclusion, This is a strategic move from all parties. Sadly, for the US, they had to take this route despite supporting them for decades. They want to ensure no such misuse happens from now on, so they are limiting access by imposing additional restrictions(Stopped training, spares, etc).