She's the antagonist from the second arc of the official comics. Rarity gets possessed. The ready of the mane 6 and Spike try to save her. Luna deals with the whole nightmare thing. Ponyville under attack. The usual.
Agreed, though I guess it could be interpreted as crippling lack of self-confidence, thus allowing the Nightmares their foothold in a way not unlike Luna's bitter envy re: ponies preferring the sun. Perhaps the key to undoing this involves some means of reaffirming Original!Rarity's worth to others.
I would just tell her to calm down and not blow things out of proportion. ... Haha, just kidding. That would be a terrible solution, and telling a girl to 'calm down' never works.
She needs to be reminded of what kind of pony she is: a very hard-working, strong-willed, independent, beautiful mare. She's important to all of her friends and to the citizens of Ponyville for being the only one in the entire village with a sense of fashion.
So, basically... I'm agreeing with what you said. Every person, ponies and humans alike, needs to be reminded of their importance every now and then. It'd be a shame to see Rarity, of all ponies, be consumed by her nightmares, which is why I treated this entire Nightmarity concept as a joke. I have my complete trust in her and the community that surrounds her that things would never stoop this low.
Well, the comic's one of those continuity spinoffs aimed at slightly older readers, so I can see why they'd go for the Pony!Venom development. Corrupted/mind-warped heroes are a handy drama-generating formula, after all. At least Nightmarity's shenanigans are unlikely to balloon into anything quite so overwrought as the X-Men's "Dark Phoenix" arc.