Check out STT's SB sealer (we used SB Deluxe). We used it on two types of honed travertine, and on a split-faced travertine mosaic, and it didn't darken any of those. We also used it on White Thassos marble, I'm pretty sure it's polished, and there was no change in that either. If you call the company, the owner (Stephen) is very knowledgeable, and very helpful. The sealer is practically odorless, and extremely easy to apply and then buff off (any excess). I have no affiliation with the company whatsoever, BTW, just a happy consumer! Here is a link that might be useful: STT sealers

I can’t find any reviews of it online plus you can’t buy it anywhere. Amazon say it’s currently out of stock and it has no reviews. Since we have to regularly seal it I would like to start of using one that we can also easily purchase.

"So the 3' limit for span of 3cm stone is not applicable here nor is the requirement for support of cantilevered stone since the stone isn't cantilevered" It certainly is. You've got a huge span which appears to be more than 8' which is unsupported. The Natural Stone Institute allows 3cm natural stone to cantilever 10" without additional support, not 15". I've seen stone like yours sag and when it does, it torques the stone. If you develop a crack starting at a sink corner heading to the unsupported edge, you'll know why. This isn't going to happen right away; it's going to take years. But hey, no worries, you've got a sign off from your fabricator, GC, and cabinetmaker.

If the quartzite will not accept the sealer it most likely does not need to be sealed. The sealer is just sitting on the surface and not soaking or really drying.