So now we are changing rules, because we don't want to deal with spell ambiguits.
RAW there is no ambiguity here. If you step on caltrops and fail the save (which you can choose to do) your speed is reduced to zero so you stop, period. If you have a fly speed you can't even fly further away because your speed is zero.
I'd really be careful to give the spell the extra power. Otherwise some player could try to use the spell and cancel other things. Force someone to go through an illusion.
They could in theory yes, but that is hardly an example of a powerful spell use and it is so situational it would likely never come up in play.
As far as commands go, flee is not a very powerful one. Drop is. Drop will flat destroy most high level martial-type big bads at the cost of a 1st level spell.
Or maybe allow you to move despite being affected by Otto's irresistible dance. Which spell has priority anyway?
Assuming the target is currently in the charmed condition, they both take affect, there is no conflict. The target uses all its movement to dance in place, the target must spend its turn moving away and the fastest available means ... there are no means to move, so it spends its turn dancing.
The only conflict I can actually see is with the Frightened condition where you make the target flee towards the thing that is frightening him. In that case I would suggest the "fastest available means" is whatever way takes him away from the caster in a fashion that takes him no closer to what is frightening him. if he can't move away from the caster without moving closer to the thing that frightened him then he doesn't move at all, just like if he was grappled or restrained or his speed was 0 or he was under Otto's dance.
A good question. It does circumvent fear and charm resistance.
But fleeing does not always have to be done out of fear.
The spell does not cause Frightened or Charmed and nothing in the spell says it cancels these effects either.
I can't think of a way charmed would come into play, but Frightened flat stops any movement towards the source of your fear. You can not "Flee" from a caster towards something that is Frightening you any more than you can "Flee" while you are Restrained or Grappled.
The spell can cause you to move towards something that will hurt or kill you, but it can not bypass conditions such as Frightened, Restrained, Paralyzed, Grappled etc. If it could that would make it extremely powerful -
Fighter can't get into melee with the Dragon because he is frightened. Cleric casts Command on him, he chooses to fail and runs right up to it. Or player is stuck in Terrasque's mouth, Cleric casts Command on him and he choses to fail and gets out and runs away. That would be a huge power boost to the spell!