Hi there and welcome to BYC!
I had an EE that used to get to the highest point possible and preach a sermon at the top of her lungs for about an hour before she would go lay her eggs.. Basically.. get everyone ignoring her then slip off quietly into her secret spot.
I got a Leghorn, expecting it to be the least liked bird for your stated concern reasons.. Turned out to be the bird everybody looked forward to seeing most, very stunning with that huge red comb on top of her little body.. and oh boy full of character and overall good flock member!
All chickens are individuals regardless of breed. Mix ups happen ALL the time.. not just at TSC, though they are notorious for it.. Most of their employees will ONLY be able to tell you what's on the label that came from the hatchery. People often presume a white chick with straight comb to be Leghorn.. but it could perhaps also be a Cornish cross or white Rock.. Maybe, just maybe.. it will turn out to be a pleasant surprise.. How many times have I worried about what never came to pass.. that's like paying debt you don't owe! F-E-A-R.. False Evidence Appears Real.
I DOUBT they can take back a chick.. but I'm not sure of their live animal policy.. bio security is a REAL concern for many of us!
I think you are correct that it may not be a Wyandotte at all.. noting I HAVE seen hatchery Wyandotte with a straight comb in the past. I hope you can resolve this in a satisfactory manner at the local level knowing that MOST of us really ARE trying our best!
One thing I DO know.. the gender accuracy is usually 85-95% depending on hatchery.. When ordered from the hatchery.. they will reimburse your pullet chick cost but will not replace it or take the rooster back.. THIS is where some local feed stores SHINE above multi conglomerate (slave master) corporations! TSC certainly ain't perfect.. they're like the Walmart of feed stores.. I'm thankful to have both and more hours available and for the things they do have to offer.. I sort of wish they weren't trying to expand their inventory so much.. as it's clearly diminishing worker and management moral.. if you've never been subject to the weight of the store on your shoulders.. maybe cut them some slack?!
Hope this is encouraging and helpful!
Congrats on your babies!