You are correct.
Not since the Big Bang. It started accelerating only several billion years ago: Evolution of the universe - Expansion of the universe - Wikipedia
In other words, you have found that \(v=\sqrt {ax}\), where \(v\) is speed, \(a\) is acceleration, and \(x\) is displacement. There is a known kinematic equation for this: if a body starts moving from a rest with a constant acceleration \(a\), when it moves the distance \(x\) its speed is \(v=\sqrt {2ax}\).
If the particle is in a superposition of momentum eigenstates and its momentum is measured, then its state changes and becomes one of the momentum eigenstates (physically, a narrow range around such eigenstate).
(It is after midnight here, so the follow up questions might need to wait.)
Generally, we do not.
Physically, as @swansont has mentioned, momentum eigenstate is impossible. So, physically, it is always a superposition. But its range can be very narrow, concentrated very close to an eigenstate.
This cannot be answered. What can be said is,
If a particle has a definite momentum, measuring its momentum does not change its momentum, its position, its spin, etc.
(We can call it, "particle".)
Yes, if a particle has a definite momentum, measuring its momentum does not change its state. The same holds for its position.
A measurement usually changes the state. Non-commuting implies that the measurement of one observable affects the measurement of the other. The two cannot be both measured independently on the same state.
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