I'd just buy a new hub with 10 speed free hub and have the wheel re-built at a bike shop. Shouldn't be too much labor, however if you can't match the flange size you'll have to get all new spokes more than likely. Spokes will add to the cost. I don't think much of removing material. I did machinist work for a while too. While I know it's easy to do if you have a lathe, you can't be certain the remainder of the free hub body will be strong enough to last over time.
You do have 10 speed shifters don't you? CS-6700 that you mentioned is a 10 speed cassette IIRC.
You have tried to get your money back from the seller haven't you? If eBay or if bought with a CC, PayPal or other means there is some recourse that favors you.
Can't imagine what free hub you got that won't fit anything but a Shimano CS-6700. Pics can be uploaded to the Gallery here on BF and just let us know they are there.
Welcome to BF.
Oh... The CS-6700 does come in a 12-30 if you have to have 30T or larger. These people have one if you are in the USA.
https://www.universalcycles.com/shop...s.php?id=32154
You don't by chance have campagnolo one thing and shimano other do you? They won't go together. Though I'm stumped about this step on the splines unless it's the very end of the body.