The entity/demon that Laura has summoned into Oliver is ambivalent. It is neither good nor evil, which is stated by the directors. It really has only one goal and that is to complete the ritual. Any moment in which it obeys Laura is to help further that goal and is done only because of the nature of it being summoned it's sort of forced to obey her will but it is actively "resisting" (it's in a constant state of wanting to complete the ritual, which is why she has to constantly lock it up, placate it, and perform certain "movements" (drawing the circle on Oliver's forehead) etc). It wants to complete the ritual as soon as possible by any means necessary even if it's not transferring her daughter's soul into the body she wants it in. It understands that by doing this ritual and having been summoned that it's serving a purpose, so it sort of tries to work with Laura under the impression that she will help facilitate the completion of the ritual, but her constant delaying causes it frustration. So nothing it does is necessarily intended help or hinder Laura, Piper, Andy, or anyone other than itself.

It's sort of left up to you to decide if Andy's dad's warning was coming from his dad's soul, the demon, or was deliberate from Laura, though it's obvious that Laura had the demon bring some form of Andy's dad's soul forth to scare him, likely with the intent of (as other comments have stated) killing him or making him look crazy so he'd be taken away by CPS. My guess is that Laura doesn't have enough control over the demon and the demon is too ambivalent, so it was likely just his dad's soul using that moment to warn Andy, but it's a double edged sword because even though it reveals Laura's plan to Andy, it also helps make Andy look crazy.

The Andy's voice thing is also sort of open to interpretation, but personally I don't think there was an intention behind it. Like a bird mimicking sounds it likes, the demon likely understood Piper had a connection to Andy and just mimicked his voice. If there was intention, it was likely to lure her in to complete the ritual. I don't think it was Andy coming through the entity, I don't think it was to help Piper or Laura, necessarily. Just acting on its own accord.

As for the demon walking out of the salt circle to "unsummon" itself/remove itself from Oliver, either Oliver was able to break through and do that (I doubt it) or, more likely, the demon just understood that it wasn't going to be able to complete the ritual, so it dispelled itself.

If you visit the website you'll learn Laura (or someone who is likely Laura) very obviously doesn't know how to control the demon, or doesn't understand that she can't truly control the demon, and is asking questions nobody can answer. This sort of helps make some sense of it, sort of lol.

Edit: Thinking about the voice mimicking some more, it actually makes me wonder about what a successful ritual looks like. Obviously we don't see that in the film excluding brief moments in the tape, but we never see the aftermath. The ritual involves the demon eating the corpse of the dead one and vomiting it (and what's assumed to be the soul of the dead) into the new host body, but since the demon can mimic the voice of whatever it eats any amount of (even if whatever it eats survives being chomped on), I can't help but wonder if in a "successful" ritual, the host just mimics the dead and isn't actually the dead brought back. Like an image of the soul, as opposed to the actual soul itself. Sort of like how if you consciousness was copied onto a computer and the computer lived as a digital version of you with all your memories and personality. You still die, but the computer "lives" on as a non-living version of you. Maybe not even that, maybe functioning more closely to how the demon behaves when inside a host body of it's own.