It's not just about people appealing to an authority on a different topic, but people rarely point to Carl Sagan to prove something in D&D is true, so it doesn't come up very often. It's most commonly people pointing at an authority and declaring their position to be true because an authority said it, which is also an Appeal to Authority fallacy. This is a fallacy, because one of the hallmarks of an Appeal to Authority is if real authorities are in disagreement about the topic, which is pretty much every topic in existence.
No, it isn't. And as for 'pretty much every topic in existence' being in dispute, I heartily disagree. Most things are settled matters, or else fall into the category of the unknown. Some things are known to be unknown, others are unknown unknowns, but very few things are thought to be known, but are actually unknown.
You are falling for the tactics of cheap rhetoriticians and populists here, who thrive on the creation of a false sense of doubt in their audiences, and then ironically themselves pose as a source of expertise! I won't go into any examples as they're both obvious, too numerous, and inflammatory.
They are paid to testify truthfully under oath with their expert opinion on the matter. Being paid doesn't(or at least shouldn't) matter.
Ah, you might think so, but that's not the case. Certainly no expert will testify contrary to a position which can be colored to be defensible. Perjury is a very high bar. Beyond that, most testimony is itself examples of various forms of fallacy (or to view it another way, rhetorical tactics) in action. Having an expert testify to something which sounds impressive, but is actually irrelevant, is for example quite common. Also, remember, a decision in a court of law requires a high bar, even in civil cases. All an expert need do is sow any seed of doubt to be effective with a jury.
Again, law and legal testimony by experts bear very little resemblance to scientific process or standards. I once helped assemble MANY cases for the EPA in superfund litigation. As chemists we could show facts with essentially 100% certainty, yet industry had other chemists, sometimes ones that also worked for us, who would happily testify to the opposite. Everyone KNEW they were wrong, but no court is going to waste its time trying to prove that.