Thanks for the detailed reply. I won't comment on c) because I haven't researched it enough yet myself to know what or what not to believe. But let me ask you more about this that you wrote, "The Trump administration believed the entire Russian collusion investigation was politically motivated and unwarranted. He absolutely resisted and tried to obstruct the investigation. There are 10 clear incidents where this is the case."
Is it your opinion that it's OK to obstruct justice when a person feels an investigation is unwarranted?
Also, I think this comment of yours is quite wrong: "Hence the underlying criminality of "collusion" was shown to be a false allegation." It wasn't shown to be a false allegation. Rather, not enough evidence was found to SUPPORT the allegation. Those are two very different things. And personally, I think it's very possible that Trump's obstruction may very well be the REASON they didn't find enough evidence to support the allegation. But it is what it is, and I'll grant that a criminal conspiracy was not proven to exist. (But it wasn't proven to not exist, get it?)
I will also point out another comment of yours that I think is wrong: "The attorney general concluded that they don't constitute obstruction. Hundreds of lawyers are now writing in and making an argument that they do constitute obstruction. The argument becomes moot as a sitting president cannot be indicted." A sitting president cannot be indicted (as per policy) because it is felt that it's Congress's duty to first impeach said president and remove him from office, should the offenses rise to a certain level. Therefore the argument is very much NOT moot, in that Congress now has a duty to determine whether Trump's obstruction is an impeachable offense. If the 900 or so lawyers are correct, then in my opinion Congress should certainly impeach and remove him from office. And then he should be indicted.
Regarding the line in bold, I start with the premise that politics is a very contentious and dirty business. Both parties in this election were using derogatory language, spin and underhanded tactics. In that context, and in the context that Trump felt he was in the right not to comply with the Mueller investigation, it becomes a question of degrees rather than a moral absolute. Besides, Trump didn't just "feel" he was right, he "knew" he didn't collude. For example, making a speech that says, "I hope the Russia releases Hillary's emails," is crude, but it doesn't constitute collusion with Russia. Neither does being informed that wikileaks is going to release information on Hillary. A meeting took place at Trump Tower and Russia might well have been discussed, but Russian interference didn't require the Trump campaign's involvement. The evidence on collusion turned out to be pretty weak. Enough so, that the attorney general proclaimed that Trump was being "falsely accused." No, Tump can't 'disprove' that collusion didn't take place, but in realty, there's no way to do that (prove a negative), and Trump doesn't have such a burden of proof. It would be like accusing someone of sexual assault nearly 30 years after the fact, providing no evidence--not even a date and location, and then telling the person accused he needs to prove it didn't happen. That's not how justice works.
Regarding your second statement, I'm looking from the attorney general's perspective. He is presented a set of facts gathered in the Mueller report and must make an analysis in a purely legal context. Is there enough evidence here to successfully bring criminal charges under collusion and/or obstruction statutes? The attorney general's answer on both counts was no. Amidst all the accusations that Barr is a liar, or traitor, or is hiding, or is acting to protect the president, I hadn't yet seen anyone actually attempt to refute the merits of his legal conclusions, that is, until this group of attorneys sent a public letter in the last week to make an argument that obstruction could potentially be criminally litigated (were the defendant not the President of the U.S.).
Regarding your third statement, yes, the Congress can consider impeachment, and the Mueller report may have opened the door to explore impeachment. So far, the Democrats haven't rallied enough support to pursue it. The Republican Senate would also likely block impeachment, as for the most part, they support the President. If the Democrats aren't going to pursue impeachment, they don't have anywhere meaningful to go with this, other than to drag out more hearings, issue subpoenas, hold people in contempt of Congress when they don't participate, and complain about it all.