Yep, I did buy some Kwiat studs that were platinum, and all but the backs WERE platinum. But when they showed up, the backs were gold, and I couldn't do anything with them without a platinum backing. I recently ended up returning them because even with the platinum backings I had issues and never ended up wearing them. I had a set of platinum studs done elsewhere and so far have had no issues.
Personally, I am VIOLENTLY allergic to gold, doesn't matter how it's alloyed. It cannot touch me or my skin literally starts to boil. People can absolutely be allergic to gold, platinum, or any metal at all. And they can also develop over time, where you never were and bam one day you are. Plating doesn't always work (as suggested above) but for some people it can be a temporary fix. Nickel is also a problem for me.
Base metal and nickel based things can be very big issues for metal allergies. I am wary of most stainless because in the US there's no rules about how much Nickel can be used, and in other countries there are.
As for the question about nickel in wg: yes in the US that is the main alloy to make it "white." Nickel WG is usually pale yellow and requires the rhodium plating to be white, and that plating will wear off over time. Palladium (in the plat family) is the best alloy for gold if you want it to literally be white and not really need plating. It gives it a more "Gray" tone, but it doesn't require plating. For many people this is the ideal mix for white gold, but it won't help me a lot.