If an exerted permanent is already untapped during your next untap step (most likely because an effect untapped it), exert's effect preventing it from untapping expires without having done anything.

If you gain control of another player's permanent until end of turn and exert it, and then that player regains control of it, it will untap during that player's untap step.

You can exert Sandstorm Crasher as you declare it as an attacking creature. You can't do so later in combat, and creatures put onto the battlefield attacking can't be exerted. Any abilities that trigger on exerting an attacking creature will resolve before blockers are declared.

The token created by the reflexive triggered ability copies exactly what was printed on the original creature and nothing else (unless that creature is copying something else or is a token; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on. If it is a Vehicle, it is not crewed.

If the copied creature is a token, the token that's created copies the original characteristics of that token as stated by the effect that created the token, with the listed exceptions.

If the copied creature is copying something else, then the token enters as whatever that creature copied, with the listed exceptions.

Any enters abilities of the copied creature will trigger when the token enters. Any "as [this permanent] enters" or "[this permanent] enters with" abilities of the copied creature will also work.