why does it take the national mediation board years to release unions from stalled negotiations? how did it get like this? what do they actually do?

Because billionaires control the government.

Their number one goal is to prevent interruptions to air service. Releasing unions is counter productive to their charter.

This is a joke, right?

Nope. New to NMB relations?

Quite the contrary. I am retired.
I will go easy on you here because I can tell you are just a young feller.
The idea that the NMB thinks they have a goal is precisely the problem.
The idea that you (and others) think them having a goal is OK is precisely the problem.
Over the years the NMB and its "Mission" have been contaminated by politics and assumptions such as yours.
Unbelievable.

You are asking the right questions.
But you have to be willing to dig a little.
You suspicion they have morphed; you are correct.
Where they are today is NOT what the original intent(ion) was.
The reasons and explanation is pretty complicated though. Not enough room here.
Suffice to say politics and acquiescence (by ALPA and others) has led to the monster we have today.
Please understand the population that has to deal with the NMB as it relates to the RLA is quite small when compared to labor as a whole.
But your gut is correct; they are steamrolling you and all the RLA unions.
This is NOT why they exist.
They (labor under RLA) will have to stand together to reign in this "power trip" they (NMB) have been on since the mid 60's.
Good Luck and keep asking those questions and don't let these youngsters who act as though they know deter you.
Otherwise you (the plural) are doomed.

It appears that he is correct in his "idea"

From nmb.gov:

Pursuant to the Railway Labor Act, NMB programs provide dispute-resolution processes to effectively meet its statutory objectives: avoiding interruption to commerce or to the operation of any carrier;

Like it or not; the RLA provides for protection of the national transportation system; which by default, favors management during labor disputes and negotiations.
I don't think that any of us think that this is "ok" but we have no choice but to play by their rules.

Well it appears I have some research to do.
This statement you quote is NOT consistent with the original "mission" or "mission statement".
If thats the case then it would appear that politics has become so influential as to have changed it's mission.
Not sure when that may have occurred. I do know at one time changes to NMB policy were followed along the same lines as changes to other government organizations. In the same manner one would follow changes to DOT or the FAA that would affect operations.
As I said I'm retired so I have the luxury of time to look into this.
What a travesty though.
The intent was NOT to infringe upon the rights of workers (labor) but simply to ensure any impasse was not arbitrary.
The powers that be didn't want minor disputes to interrupt commerce.
They had specific instructions to promptly determine whether there was room to negotiate or declare an impasse.
Yes, there is a history here but we (labor) should have never allowed them to morph to this level of control.
This was reemphasized to them in the late 60's after they limited strikes to basically 1 airline at a time. (and releases timed to congress).
Still it is important to remember and fight for the fact that while they refuse to release you once an impasse is determined they are in fact abridging your rights, regardless of this superfluous language that has been added to expedite a political "goal".

I am so confused.