I use vinegar, but am sure the alcohol is fine. I've read with vinegar to remove it as it will damage the grout. I never do, just forget. I'm sure the givee will love it for many, many years.

I understand your angst, but color inconsistencies are really a non issue. Over 32 years I have installed many tile applications and have never had a client find an issue with grout color variation. More important is the sanded/non sanded. 1/16 really needs a smooth grout unsanded, and you should avoid certain grouts that are "hybrids" - grout for thin joints that has fine particle filler. You can never get them to have even suspension through out the mixture.

I've only used the pre-mixed grout a few times, not a big fan. The instructions for cement grout warn about color variation but, in 20+ years of installing tile, I never had an issue. You can get color variation if you add more water to the grout after it's started to set up. The bigger concern there is that it will weaken the grout. Another cause of color variation is using different bags. If the job calls for more than one bag of grout, you're supposed to "box" them (mix them together). A kitchen backsplash is unlikely to need more than one bag (probably only a quarter of a bag.) My advice would be to use un-sanded cement grout. For 1/16th joints, add water and mix it until it's about the consistency of peanut butter or just a bit wetter. Let it set for about 5 minutes to "slake" then mix it up again. As for the brand: PolyBlend by Custom, Laticrete, and SGM are a few I've had good luck with. I've heard but never saw first-hand that Mapei had color variation issues. If you have to use the pre-mixed stuff, it won't scratch your marble. Pre-mixed grout typically contains sand (silica) which won't scratch the marble but will be difficult to get into 1/16th joints so you will probably end up with several unsightly holes in the grout that will need to be touched up.

Here is a link. It's for a specific company but should give you some guidelines on care and maintenance of your tile. https://glaze-n-seal.com/seal-maintain-saltillo-mexican-tile/