I don't think anything he's said so far is a spoiler, but I can see your point.
I am an avid scholar of Norse mythology- it occupies a significant portion of my life, and I'm glad to help. However, up until now it just seems like you're just posting information and I'm not sure what, if anything, you need help with.
A note on Svartalfr- there's a bit of a lore debate still going on with these guys. It's pretty clear at this point that "Svartalf" refers to the dwarves, but there are some who believe there is a separate race supported in the sagas of "Dokkralfr," true "dark elves" that are counterparts to the Alfar. They would have their own realm, explaining why there are attested to be Nine Worlds, and yet most sources (including your post) only name eight. As for what the Dokkralfr would look like... there really isn't anything in the lore describing them, so I guess it would be up to the GM to fill in those gaps.
Also, about your discussion of giants. In Old Norse, -jur (pronounced "yoor," just as a note to anyone reading) is just a suffix that takes a male noun and makes it female. So you could add -jur to the end of any of the jotun tribes to describe a female of that race and, probably, remove it to describe a male of the gygjur or ividjur.
For example, a female of the Aesir is called an Asynjur, and a female of the Vanir is called a Vanjur.
Reversing that rule with Gygjur and Ividjur to get the male counterparts, we'd have Gygir and Ividir.
That's more of a linguistics answer than a lore-supported answer, but for a roleplaying game I'd say it's close enough.