would I need to add a heat pack or some other temporary heating in her transportation cage?
i don't think so.

If she stops sneezing by the time I return, should I take her to a vet anyways?
a lot of people will say yes. if you keep an eye on her and she does not do it again you should be fine.

I've lost two pets from unpreventable genetic conditions this year and I'm honestly terrified she might get really sick while I'm gone. ;-;
really sorry to hear that.

However, could being away from her for this amount of time seriously worsen whatever's going on if it doesn't clear up?
she should be fine for a day and 1/2. keep a close eye on her, if things worsen or continue it will be vet time. even if it comes to vet time she will probably be fine good luck and don't stress too much!