Legally, it's not relevant whether the act was legal, as long as it had the effect of impeding the investigation, or was done with that motivation. Obstruction of justice charges were brought against Clinton under circumstances which were at least as dubious. I'm not actually suggesting any claims other than that charges will likely be presented; it's just as likely as not that this will be done by one of the Congressional probes if it's not a direct result of the Mueller investigation. Whether the charge would be justified is a question beyond my competence, and as I said, at the moment I doubt a conviction.
Trump cannot directly fire Mueller according to the law. He would have to either get Rosenstein to do this, or fire Rosenstein and get the new deputy AG to do it. This would probably not go unnoticed -- you may recall a certain "Saturday Night Massacre" which got some press attention. If Trump were to directly fire Mueller, it's just as likely as not that this would be upheld in a court of law, but it would also trigger an immediate Congressional reaction, and it's unlikely that the legality of the action would be directly examined.
There can be and wasn't any obstruction of justice involved in the firing of Comey....
As you seem to have said, there can be obstruction. Whether or not there was, is a question which neither of us can guess.