Except that being at a different location in time does not imply being at a different location in space, necessarily. Here on the surface of a planet, it certainly does, but not necessarily in the abstract discussion of the nature of motion.
Well, surely it does imply different locations if time and space are one and the same, or to put that in better language, coordinates of the same field.

We can still have our 3 spacial components as 0-vectors, though, showing that motion in time, but not space is possible.
But that 4th dimension of time isn't a 0-vector. So there is always motion through spacetime.

...inertial mass and gravitational mass... they don't have to be the same.
Sure they do. Gravity is acceleration, it's basically resistance to a change in state of motion caused by this acceleration. How is that not exactly the same as inertial mass? Gravity just does the acceleration for us.

Surely you at least expect it to be the same? I mean isn't this what GR is? An attempt to unify graviational and inertial mass? That's my interpretation of GR.

Whether or not the future exists before we arrive in it seems unmeasurable.
Surely we only need to prove space and time are essentially one and the same? Because once we do that, then how can we arrive in a pre existing region of spacetime? Something else existed in that part of space at that moment in time.

If space and time are seperate, then ok we can be in a region of pre existing space. But if we can't say space without actually meaning spacetime, then how can the region of spacetime that I am approaching already exist? If it does, why can't I observe it? One nanosecond in the future isn't that far away, but every direction I look is in the past. Where is one nanosecond in the future? Why can't I see it?

And of course, we can measure a great deal that we once would have called "immeasureable". What does atom mean? Indivisible. When we named the atom, it was unthinkable that there was anything smaller, let alone that we could measure such things.

I'm not sure how one could measure something "in the future" in that regard.
What does it tell us if we prove that we cannot measure the future?

The many worlds interpretation of QM would imply that all possible futures exist, but that's not science
Yeah I don't get on with this. I'm happy enough to say all possible futures have an equal probability, and this one is happening.

You're kind of on to something big, though. In a very real and practical sense, the reason for gravitational attraction toward massive bodies is due to time dilation
This only strengthens the idea that space and time are the same thing, in the same sense that electricity and magnetism are the same. If space is curved, then so too is time, to a proportional degree. If this is an unavoidable truth, then space and time are one, and motion through spacetime is, for everything with mass, into the unseen.