Well I'm kinda sure Karass didn't live during the aeon of strife. But it's cool to see a concept for him, however the concept seems overfilled with stuff that makes almost no sense (at least to me) xD However I quite like the style with talents like in Heroes of the Storm.
Yep, they are also the Zerg from Heroes of the Storm map Braxis Holdout.
I think you've got the Protoss wrong. They are not mean to be Xel'naga (Despise the name Ihan-rii, which means "Great Teachers" and is another name for Xel'naga, but I'd say it's just a poorly chosen name, also for Zeratul's coop units ... giving them the name Xel'naga before the unit name was really stupid imo), they are ancient Protoss, who somehow can use Xel'naga technology and use it to make their own "constructs" or units or whatever. The lore explanation is that most of them went to stasis, but they still had Observers watching over the Koprulu, so they learned about all the tech of other Protoss factions and kinda replicated them with the Xel'naga leviatating stone technology. It's more likely they would create their own units, but it's plausible explanation for skin set.
I was a bit confused too at first, but after reading the lore of each of the skins and this summary on wiki it all started making sense. Even tho this faction should be ashamed for not helping against Amon (Except for alternate reality where Zeratul survived) as they seem to be the most powerful Protoss faction, because of the usage of Xel'naga tech.