So, Nien Nunb (crew) clears stress on an ionized maneuver, he says: " You may treat all [straight] maneuvers as green maneuvers."

R2 Astro says: "You may treat all 1- and 2-speed maneuvers as green maneuvers."

Pretty much the same, right?

So the real question: AP-15 is ionized from Pulsed Ray Shield regen. Next turn he does a 1 white straight from the ionization, and R2 astro turns it into a green maneuver, and he clears his stress.

Sound correct?

Yes, that's correct.

Correct. The same goes for the Nien Nunb crew card.

You are correct the Astromech makes the ion move green.

To go a little deeper though just for clarity, it works because the rules for ion just say "perform a white 1 straight". It's specifying a particular maneuver with a difficulty rather than modifying an existing maneuver on the ship. If ion instead said something like "Perform the lowest speed straight maneuver on your dial and treat it as white" then Nien and the mech would NOT work because if multiple modifications are applied to the sake move the more difficult one takes effect. So if you get Damaged Engine (all turns are red) on a Y-Wing with Unhinged Astromech your 3 turns go back to being red because the negative mod overrides the positive one.

Just to make sure that I have this correct,

So as long as the ability trigger is perform the maneuver instead of reveal the dial the effect will work.

Fly casual.

It will work for any 1 or 2 speed move as long as that move hasn't been changed to a colour other than green.

If it started white or red, it's fine.

Slight caveat to the above: the one move you cannot change the difficulty of is the penalty white 2-straight for revealing a red maneuver while stressed.