Yes, it is. It tries to say "stand over there" implying the action of moving over to the mirror and stopping there to stand there. That's what the Hungarian sentence means. I am standing somewhere or sitting somewhere or whatever, and now I am going to walk to the mirror and stand there.
So in this case állok implies the first person is standing upright, and the allative hoz implies motion towards i.e. motion towards the mirror in an upright manner. In that case I think the best translation into English is either
"I go and stand at the mirror" / "I go and stand by the mirror"
"I go up to the mirror" (no need to mention stand here because to go, it is assumed you are upright in order to go)
I think it is confusing for an English speaker because "to stand" is only ever used to indicate "motion up from a seated position" or "the state of being upright yet otherwise motionless".
From my understanding from other threads, "állni" can be used with co-verbs to indicate motion followed by a stationary phase e.g. driving up, pulling up in a car, parking etc , is this correct ?
Yes, it is mostly correct. But it does not indicate the method, or the manner, in which I am moving to the mirror. It only says that I am moving to the mirror and, when I get there, I will be standing upright (as a person). I could also sit or lie to the mirror. Again, moving there (practically walking) and then assuming the specified position. And yes, you can "stand with your car into the garage", i.e. move it to the specified target location and place it in its regular stationary position.
Ok so in that case "I go and stand at the mirror" is probably the best English translation because it doesn't imply anything about how I go there (e.g. I could go up in a wheelchair and then stand once I get to the mirror). Whereas "I go up to the mirror" could imply I go there on my hands and knees and remain there once I reach it!
Sometimes when I am standing in front of the mirror I might just say "I am standing in the mirror", or "I am in the mirror". This doesn't make literal sense but the reasoning here is that my reflection itself is positioned within the frame of the mirror. I recall telling my son to "go stand in the mirror and see if your shirt fits". Any other English speakers here say it this way?
I. Sure, but they are translatable into each other.
II. I think more important than Duo's acceptance of your translation is that you understand what the exact choice of words is doing to the meaning of the sentence. I tried to address that in my recent reply.
III. Thanks, I appreciate that.
IV. I'm fine with your translation, I just added some explanation. My comment wasn't even addressed to you.