Is space time a Kantian concept – that is, just a concept of the mind at work? That is that there is no ‘out there’ – it’s in the ‘mind’?
Relativity might work but it certainly requires a stretch of the imagination to accept for example that the speed of light is constant. Think about that, that if we rode on a photon all of the past and all of the future would happen at once. Sure it is all mathematically sound, but does that really mean that there is something ‘out there’ as Kant’s conceptual world?
And if you don’t agree, then why is light singled out for this special, and very odd treatment?
Did Einstein place space time ‘out there’ and with that acceptance create the problem of non–local phenomena like entanglement? What about the Quantum Zeno Effect?
Was Einstein with all his brilliance casting a shadow over quantum mechanics, after all Max Planck once said: ‘This mind is the matrix of all matter.’ While Schrödinger asked ‘Who are we?’
Give a physically distinguishable definition of "out there" vs. "in the mind" and we can try to discuss this further.
As jinawee comments, there are no frames of reference that move with the speed of light, since the photon we "ride on" would have no speed at all by definition of a comoving reference frame, and that contradicts the constancy of the speed of light in all frames.
I think this amounts to asking why the laws of physics are, well, the laws of physics, which is a debate I think you should take to the philosophers. That the speed of light is the limit, that the metric of spacetime is really to a good approximation the Minkowski metric is really just the way the world is.
Again, what is "out there"? Also, what about entanglement and the Zeno effect? They are phenomena of quantum physics, which can be empirically tested. How could Einstein create them?
Einstein didn't like quantum mechanics, that much is true. He disliked it for its non-determinism, famously saying "Gott würfelt nicht" ("God does not play dice"). Discussions of the mind/consciousness and of the nature of humankind are off-topic for this SE, as they are not part of any physical theory. I again direct you to the philosophers for this one.
(1) Spacetime isn't all in the mind. Spacetime is just the gravitational field and it has degrees of freedom independent of your brain, so it's not all in your mind. You can't make an even happen on Tuesday just by thinking it happened on Tuesday.
(2) As commented by others, you can't ride a photon. Also, the way to figure out whether something is real is whether your current explanation of the world says that it's required to explain stuff. If so, the best guess is that it exists.
(3) Why is light singled out as having the highest speed? It's not. Any massless particle travels at the speed of light.
(4) Einstein didn't make space and time independent of our thoughts, they already were independent. Entanglement is not non-local, common opinion to the contrary notwithstanding. As for the Zeno effect, it is an example of something that happens to quantum systems when they interact with other systems in certain ways. This happens independent of what you happen to think about it, so yes it is "out there."
(5) The fact that Einstein misunderstood quantum mechanics and how the world works can hardly be an argument that casts doubt on the idea that the world exists independent of our thoughts.