>How long is Karkat’s lifespan? He's not on the hemospectrum at all, so...

We actually have no clue and the comic never actually says how long Mutants can live lol. Though they do say Mutants don't live for very long, that's not because of their natural lifespan, but more so because of the Alternian drones that cull weaker blood types. So technically Karkat could live for a pretty long time, but I think he just has the same lifespan as an average human lol.

>What is Jack's Real Name? What is Scratch's Real Name?

Jack's real name is "Spades Slick". We don't know Doc Scratch's "real name" but I always thought he was just lying and that he either has no name at all since he's just a vessel for Lord English or his name really is just Doc Scratch and he's a liar lol.

>What is the Ultimate Riddle?

When Karkat and John have a conversation here about the "Ultimate Riddle" they're basically discussing a "what came first, the chicken or the egg?" kind of riddle. Karkat is saying that the game, Sburb, lays out a bunch of clues and puzzles for the players to solve that eventually lead them to the Ectobiology lab to create themselves, and that it's supposed to reveal the cyclical nature of the game itself, how there was never an escape from playing because you were always destined to play this game since you are the only reason you even exist in the first place.

>Can trolls not handle any sunlight at all, or is the Alternian sun particularly strong? Like, does Karkat avoid the Earth C sun because he'll literally burn up, or just because he’s Karkat and doesn't like outside?

In the game Friendsim, we actually learn that the Alternian sun is incredibly strong and that it literally scorches the skin off of Trolls who stay out too long. Trolls keep their Hives dark on purpose because direct sunlight kind of hurts their eyes. In the Epilogues, Dave mentions that they always keep the "Hive" (or House) dark because Karkat doesn't like having all of the lights on, unlike humans. So yes, Trolls don't really like sunlight. There's also just the added fact that the "sun" isn't "his sun" as in it's not the Alternian sun he's used to. Weirdly, he is also a bit homesick </3

>Why does Dirk get deja vu about uu? (I'm gonna try to answer this one as best I can lol)

Prince of Heart stuff. He's not actually getting Deja Vu here from a previous conversation but rather it's complicated weird "Splinter" stuff. This is a reference to the scene in the first few acts when Dave and his Bro fight on the roof. This Dirk isn't the same as Bro Strider, but he's still "A Dirk". When he says he has Deja Vu, he's talking about that other Splinter of himself, Bro Strider. He feels like he had that conversation before. He technically did, just not in this universe.