HelpFeral, I am no expert, except through my own experiences with a feral Mom & kittens, and many past stray cats (abandoned pets) I've taken in, so my advice is not The Law.
You have some time to work with her before your move date. This is good. I'd not make any decisions for a couple/few months, because Mariah is so new to her home, it's hard to know what to do based on her current state.
On a companion:
First, I'd ask you: do you know if Mariah came from a feral colony? If so, then she is used to having other cats around her, and that would be a point in favor of getting a companion for her. However, if she was a loner, another cat in what Mariah is only starting to think of as "her" territory might stress her out even more.
Another point in favor of a companion cat is if you (or another human) isn't going to be around all day and she would be left alone - I have found that, even though cats adjust to being alone, mine have all seemed happier when they have a companion. Again, though, if she isn't coming from a cat colony background, I'd wait at least until Mariah is more comfortable with her new surroundings before introducing anything else new, whether it meows or not.
However, in the short run, I'd say, no, no companion until she relaxes a bit more.
On relaxing a bit more:
Even though they are independent-minded, cats - especially insecure ones - do really well with a regular routine. Even when my Mom cat and her four kittens were all in the Hide From The Monster stage, they knew perfectly well what the regular meal times were, when the regular sleep times were, and even when the regular "Oh no, The Monster is talking to us" times were.
I caught on to this when I overslept one day. Normally, when I was in the kitchen dishing out the chow, the Family would hide under the bed, only coming out to eat when I'd left the area. One day, I overslept, and when I went into the kitchen, they were there, looking at the empty food bowls...THEY knew there was supposed to be food there! Of course, away they ran when I came in...
Point is, Mariah is trying to get a feel for her new environment, and a schedule will help her feel more like she knows what's going on. I'm offering this for what it's worth because you DO have a time constraint operating, so the faster you get Mariah relaxed, the more time you will have to decide about a companion cat before you have to move.
I look forward to hearing about her progress, and also reading whatever other, more expert people have to say on your dilemma...