1) So did everyone before her. As noted in news articles, Colin Powell also used private email for much of his Secretary of State functions including emailing foreign government officials. No-one is calling out that as suspicious. After that, Condoleeza Rice apparently didn't email anyone for her entire tenure as Secretary of State. So she's off the hook. John Kerry is, according to articles, the first ever Secretary of State to extensively use a government email. So it's not like Hilary's actions were some abberation from normal procedures. Doing it properly is the modern abberation.
2) doesn't make a lot of sense since people aren't limited to one email address. If this was planned malice, she could have used her official email most of the time, but used a third party service only for the "off the record" stuff. There would be almost no leads to sniff anything out then, she could just deny any communications even took place.
All up though, I think this was a huge failure on her advisors part. She's not a techie, she should be being advised on technical matters and compliance and when the rules change. Hell, my college advises me on technical matters, why didn't the state department have mechanisms to advise it's officials of the technical stuff?
This would not have been something that was in her field of previous knowledge, so there's that: there was no precedent nor censure for a Secretary of State doing things any differently in the past. What I think most likely is that she used her private email because of familiarity, and pre-existing relations she had with other diplomats. She had a diplomatic presense that precedes her time as Secretary of State, so she may have sought continuity of communications with many parties, rather than switching to a wholy new email address.