No, as eskimospy already pointed out, the full content of the emails was not known at the time the letter was sent (at least not officially). Once they determined that the emails could be relevant to the earlier investigation, they obtained the warrant and sent the letter to congress to make them aware of the development.
That depends on the content, and it also depends on wrongdoing on who's part. For example, if huma said under oath that she never used the laptop for her emails and then it turns out they are on there, then their presence is possible evidence of wrongdoing (could be perjury).
There was no fishing involved. They looked at emails on his laptop as part of one investigation, and saw that there were emails that were relevant to the other investigation. They then obtained a warrant to look through those. No allegations are needed, it's simply a warrant to see the content of those emails as part of their earlier investigation (which is still open). In other words, by the book.
Under normal circumstances, "may be relevant to" isn't grounds for a warrant.
I've been supportive of Comey up 'til now, but the more I look at it the more I see it as a boneheaded play, at best. He just staked his job as head of the FBI on finding some actual wrongdoing from emails that nobody in his dept had (supposedly) read. Even if he does pull that rabbit out of the hat the notion that the rules of evidence were actually followed will be questionable at best.
It reeks of poor judgement.