The backsplash will more than cover 3/8" . Most thin tiles are that by themselves. Thicker tiles can be 1/2" or more. Then the thinset is anothe 3/8" at least. Don't sweat it and leave it alone.

On the other sidde of your question, most tile used in backsplashes is 1/4" or 3/8" thick, plus you have the thinset behind that. You can probably allow 3/8-1/2" for your splash, but I agree that 1/4" variations are probably more than norm that unusual. Hope you love your marble. I thought I was going to have to tell you that I had the first stain on my marble, but I think they were actually on the sealer. They came right off with some cleaner add I will reseal the counters soon -- it's about time anyway. I think you are more likely to love them for several years and convince someone else seeing them in a used state that marble isn't so scary that to lose a sale. Worst case, someone wants to replace the counters -- or get Dry Treat applied. ;-) For now, they are yours to enjoy. I like your slabs. The speak to me -- kind of like mine did. Mine had dark lines and squiggles against a background that reminded me of da Vinci sketches on old paper. My son has done reports on da Vinci for school and we have enjoyed talking about his incredible visions and ideas -- and laughed about the line in "Ever After" where da Vinci takes a locked door off the hings and tells them that he shall forever be remembered as the man who opened a door. I think that set me up to love my slab, but yours has a wispy quality that is unusual and I like that too. Enjoy your counters, and if you have any concerns 3 years from now -- well, no certainty that I'll still be here or have the same email address, but I'd be happy to help you get them ready for a resale over a cup of coffee and talk about what your next counters will be (these are it for me -- DH says he's going out of here in a box -- LOL) Love your marble!

It would be very difficult to keep an existing backsplash with a countertop replacement. If you're selling the house, I wouldn't spend the money on replacing Corian with granite. You probably won't get the money back.

Get a 1/4" x 1" solid color solid surface molding fabricated and installed please. That's way too much gap for caulk.