"It's just been replaced and is leaking from the check valve already. " Correctly installed? Pipe dope is NOT required on all types of joints, can cause joints designed with an internal seal to leak.

If you would Google the part number it's pretty easy to find the info: Grohe pdf

Agreed. It wouldn't bother me either and I think the old look is neat. I have a cross handle faucet (mixes hot and cold with separate controls)) and in the last year I don't believe that I've ever actually mixed the water. Either I'm getting cold water for a drink or hot to wash my hands. And our water heater is set low so it doesn't scald on full blast hot.

Do you have an electric water heater? Copper plumbing? A fractured heating element in the WH can place significant amounts of voltage on the plumbing system. Or the dishwasher may have a ground fault when it's not operating, or your refrigerator through its icemaker. There are lots of possible sources.. If your plumbing system isn't properly bonded to ground, it can float at whatever voltage is present. It's not too uncommon for the bonding jumper across the water heater to be missing. Now and then the source can even be outside the house. In the early 2000s I ran across a weird situation where a friend's phone line read over 70 volts to ground. It was enough to give me a pretty good tingle. He called the phone company and they wouldn't believe him. He never did get it fixed.