Lawyer here.

So i think this is all insane, and a power grab, to start out.
But not because of these parts of this EO

I think people are trying to assume this says "the president gets to ignore the courts and congress", but it, uh, doesn't actually say that anywhere. I would very conservatively guesstimate at least 50% of people assume trump will go that route, and so they assume this is the method by which he will do it. But unless i missed something, he's actually said the opposite consistently - he wants absolute power over the executive agencies, but will follow court orders.

If he was going to start not following court orders, i don't think he would have any trouble saying it. I don't think he would issue an EO, either, since those can be challenged. I think he would just continue to fire anyone who doesn't do what he says, or he otherwise disagrees with, and let each individual decision spawn a new court case, rather than give a really large EO that can be challenged and give him a much broader setback.

As for this order itself, I really hate taking a side I hate here, but there is almost nothing interesting in the parts you quote:

To start - AG opinions (and OLC opinions) were already binding on the executive branch.
So they already provided authoritative interpretations of law.

Heck, the entire FBI is guided only by AG opinions and guidelines on how to conduct investigations, and has been since it's creation. There are no separate rules - it's just the AG guidelines and opinions.
(There is also a secret set of AG guidelines for classified investigations, and they release a heavily redacted version of it)

I don't point this out to say it's awesome, i point it out to show that this state has existed roughly forever. It's just not commonly known i guess.

The part about the president was also already true in exactly the way it is described here. This is what caused things like the saturday night massacre - in the end, the president does get to say what they want to happen, and what they think is legal and people can either resign, or do it. That was always the choice.

This is all secondary to whether courts can say the president/AG's authoritative interpretation is wrong - they can and do already.

Nothing in this EO says otherwise.

>No employee of the executive branch acting in their official capacity may advance >an interpretation of the law as the position of the United States that >contravenes the President or the Attorney General’s opinion on a matter of law'

This is also well within their power to request and enforce.
It was also already true in practice in the vast majority of cases, and most importantly, before the highest courts of the land.

At the highest court level (SCOTUS), the US is represented by the solicitor general's office, which is part of the DOJ and controlled by the AG. They also look at appeals court decisions and get involved where needed to direct positions.

In lower level civil matters, the US is generally represented by the civil division of the DOJ, and therefore controlled by the AG.

The only thing this order theoretically changes is to say that the agency counsel who would represent the US at lower levels for various agencies can't take positions that contravene the AG or president.

That is, the counsel for the EPA can't decide they think the president is an idiot and that they are going to take a position that is the opposite of what the president wants.

There is actually little to nothing controversial about that - they shouldn't be doing so in the first place, regardless of who is president or AG[1]. The president always had the power (though rarely exercised) to tell the EPA in the case above to change their position, and fire every single person who refused. It has even happened that they have forced agencies to change positions at the district court level, fired people who refused to follow their interpretations, etc.
All upheld since the days of the founding fathers.
There is and never was a 4th branch consisting of independent agencies.

The issuance of regulations and their interpretation to flesh out the law is something congress is delegating to the executive branch when they set up executive agencies.
The executive only has the power delegated to it here, and congress isn't even allowed to delegate significant power here.

To see why it's not controversial at all, if they moved all the agency counsel to the DOJ, you would have the same effect as this order.
This would not be illegal for most agencies, though some have interesting appropriation and other restrictions that require consulting with congress prior to reorging them and whatever.

Put simply:

Congress has the power to restrict or direct agencies through legislation (and the EO even mentions this), and the Judiciary has the power to say everyone else's legal interpretations are wrong.

In between, the executive has always had the remaining power to direct the agencies and how they operate, and have done so, just not as clearly as you see here.

What this order explicitly forbids actually has also been a practical problem before, and the AG's and solicitors end up having to explain to a judge why they changed their position from the lower level one, and get made to look stupid. Not that i believe they are doing it to fix that, but it has abeen a real problem.

So like I said, while I think there is a huge power grab going on, this part of the EO isn't it.

[1] The office of the inspector general is basically the one agency that exists in part to audit and investigate the other parts of the executive branch, and so would normally take contrary positions. But those positions are not taken in court, they just issue reports and inform congress. The agencydoesn't, and has never had, any rulemaking power (except to the degree necessary to carry out it's own function), any disciplining power, and any authority. That is why it is legal for it to be independent and why it was illegal for the president to fire the head of the agency.