Then Clinton, a terrible candidate who may have been the only person who could have lost to Trump made a bunch of errors allowing Trump's campaign to flip those rust-belt states. The DNC owns some of the blame here for their long term strategy in those states (or lack of). The RNC also are reaping the fruits of having spent years whipping up the loonies in their party.
2) Trump's platform is Trump. Everybody knows that. He doesn't have an ideology. He contradicted himself all through the campaign so any platform is moot.
3) Clinton was unpopular for many reasons beyond the email stuff. In many ways she was a bad candidate. She was an ultimate political insider, for instance. Personal concerns aside, many people who voted against her were also alarmed that Scalia would be replaced with another Souter or Kagan. Even given Trump's capriciousness, they thought a gamble on a "good" justice was better than a guarantee of a bad one.
1) That doesn't explain him beating out ~12 other seasoned GOPers in the primaries.
2) His platform.
3) Clinton was portrayed as toxic by attacks, not because she was. "But her emails" was something GOPers and DNCers had done for decades before and Trump staff continues to do today. It's not right, but it's not isolated and there are much worse things going on. It's not an excuse, but it was a distraction.