They’ve probably met given Aradia said “ He surely would have told you if he was making the trip all the way out here from his hive stem”

And when Sollux said he could only see in 2D he meant that he was blind in one eye so he had no depth perception.

Memories, and anything in the furthest ring that passes through them such as the Meteor or Caliborn’s juju

Dave’s eyes can be seen here.

You can see Dirk’s eyes in this flash.

On this page. 

Ancestors are a concept in troll society that are responsible for contributing a greater proportion of their descendant’s genes than the incestuous slurry from which troll eggs are produced. They share their descendant’s signs, horn shape, and blood color. High bloods tend to put more stock in their ancestor’s existence while low bloods don’t usually care about them too much. Because of the reproduction process of trolls, ancestors are usually long dead before their descendant hatches. However, they may still be alive if they’re a long lived caste.  

Rainbow Drinkers are essentially the Alternian equivalent of Twilight vampires. They can withstand sunlight and radiate light. They can also run very fast. Kanaya and Porrim’s conversation in the Openbound flash games confirms that the condition is directly related to jade blood. Kanaya theorized that jade bloods developed the condition after being in the dark birthing caverns for so long while Rose theorized that they developed the abilities in order to scare away predators from wigglers. 

They exploded themselves with the Tumor while standing on their Sacrificial Slabs

The Thorns of Oglogoth were what she was using right before she ascended.

No

Yes although the only one we know about is Feferi, being the Beforan Empress.