Oh boy! That's a tight spot to be in. My advice is to get your wig back asap, before you do ANYTHING to it, to check it out yourself! Then, I would look at the "synthetic" areas with a magnifyer, and if possible take some pics, comparing the human and synthetic hair, and send them to the company. Explain that the wig has a high percentage of synthetic hair, and is not what you ordered.
As far as the color, ia, the colors will look different to different people, and I suggest ordering a color swatch next time, but that's not here nor there . . . You can change the color on your own, or have your stylist do it. If the density is an issue, maybe you can then pull out the suspected fake hair, to lessen the density a bit.
But I would let them know that this is not what I ordered FIRST before making any changes. Maybe they can exchange it or maybe you need to file with your credit card.