I've been asking all along... If the investigations continue, how were they obstructed? Nothing Trump has done had any effect on the investigations. He may have asked Comey to ease off Flynn or make it clear he wasn't personally under investigation, but that's not obstruction. Asking for Comey's loyalty is also, prima facie, not obstruction!
Put on your objectivity hat if you can find it for a second. Trump asked Comey to stay on as director of the FBI, asked for his loyalty, asked for him to let the Flynn thing go, then proceeded to compliment Comey on the excellent job he was doing. Once Trump saw that Comey was not dropping the investigation he fired him. Im not saying that is lock n key obstruction but it is dangerously close. The firing was the action that could be taken as intent to slow, prohibit, discourage they continuation of an investigation that Trump obviously wants to be over. If the investigation had nothing to do with Trumps campaign then I'd agree with your point and Dershowitz's point but its more complicated than that. Do you really not see the issue here?
Once Trump saw that Comey was not dropping the investigation he fired him
That's YOUR presumption but not what Trump said. And again, the firing of Comey had
no effect on the investigation whatsoever. In fact, it created a scenario by which Comey becomes a private citizen and no longer subject to the confidentiality required of his position.
Now, put on YOUR objectivity hat... Trump fired Comey because he was incompetent and up until a few days before his firing, you have prominent Democrats on record, calling for his firing on that basis. Nothing about Comey's firing had any effect on the investigation other than to free Comey from being under purview of the AG. If Comey were knowledgeable of any incriminating evidence against Trump, it would've been to Trump's advantage for him to remain as FBI Director under the direct authority of Trump's AG. This absolutely destroys the entire argument.