1. No evidence. I'm sure she's killed someone, but the Dovahkiin has a far higher kill count then she ever could.
2. No evidence. The only people that would know the Volkihar blocked out the sun would be the Dawnguard, and it's pretty doubtful they or any angry mob they could rile up could take down the entire clan. Even if they did, it doesn't dismiss the armageddon Tamriel would go through. Remember, the sun doesn't just keep Vampires at bay; It allows life to exist. Without the Sun, no food could be grown, all travel would go into lockdown mode, and eventually the world would go into a permanent state of deep winter.
3. A. First off, you're assuming Vamperism is inherently evil, which is untrue. There's no reason someone can't be a Vampire and a good person. Being a Vampire doesn't instantly make someone a monster.
B. What?! Are you serious? That's like, the opposite if how it works. Tons of heros did [censored] things and we remember them..... Well, as heros. The Spartans were effectively a group of slavers who specialized in [censored]-fu, but most people remember them as the group who saved the free world because they fought one awesome battle.
1) It's highly likely she's killed people, she often says things like 'been a long time since I've fought, but I still know how etc'. Do remember she was part of a Molag Bal clan, last time I checked they weren't 'good' and wouldn't batter an eyelid slaughting most people.
If I remember right vampires can live without feeding but slowly become more insane. There is a specific example, google the oblivion quest 'origin of the grey prince'.
Since Serana isn't insane (subjective), it seems she must have fed. But she was locked away for a long time, so maybe when in a state of hibernation the blood thirst doesn't progress or as she's a pure blood vampire she's different, who knows.
2) You're right, the sun is crucial to the life of the lands and people of Tamriel (assuming the flora/fauna of TES have similar needs to earth). So when a small group of vampires successfully block out the sun, people will come from all over Tamriel to stop them. The vampires would have absolutely no chance. Maybe you should speak to Serana's mother, she shares the same view.
3) That's not what I meant by the heroes comment, think of modern day 'good' people like MLK, if he committed some 'evil' act like [censored] or murder before he died and people found out, his 'good' reputation would be heavily dented if not destroyed.
Vampirism is a curse created by the daedric lord Molag Bal, pretty much everyone who is a vampire is one because of the power it gives them, Serana included (proof via dialogue). I haven't heard of one vampire example that has chosen to become a vampire to spread goodness, they're mostly slightly insane and hostile.