Question about Sigma 18-35mm, f/1.8 DC HSM ART, Canon EF-S

Yes, the APS-C cameras have a 1.5 or 1.6 times crop to the full format. The lens is designed for these crop factors and is unrivalled in terms of sharpness and price-performance. The Metabones 0.64x adapter corresponds to a crop of 1.28x, i.e. more APS-H, but not quite full-frame. The lens will have an enormous vignette at 18mm, and will only be fully usable from 20-21mm approx. I don't think it's quite optimal, but then you have about the image detail of a 25.6mm - 44.8mm full-frame lens with aperture F1.4 (the crop cuts the image through the converter, but the aperture is optically enlarged). The details would be too long now and physically one could go deeper into it, but I hope the answer is enough for you to make your decision. I would recommend the 0.71 from Metabones, which is less close to full-frame, but the edges will have less vignette at 18mm and will be usable and sharp.

I have no idea about that. Sorry. Maybe you should go to a good specialist shop?