Upload & Sell: On
Obviously you should go buy a Canon 24-70 f2.8 L II to use while your 24-105 is out being looked at. And once you use that a little, you will have less worry of getting your 24-105 back quickly, because you may not need to use it any longer.
On a more serious note, it seems like you should proceed like you are thinking, and start with sending the lens in, to see how that works out, before considering sending off the 1DX. In the mean time, enjoy your new camera. After 3 months I still find it quite lovely to use, and am very glad I skipped the 5D3, and waited and stretched a lot to get a 1DX.
My trusty 24-105 is just 7 years old if I remember right, and I got it when they first came out. It has been through a lot, and still works fine. But I really am enjoying the new 24-70 II that I got, with nice image quality and that extra stop of light gathering, which not only helps your exposure in low light, but helps you see better through the viewfinder in low light, and helps give better and more accurate auto focus than an f4 lens.