Can I:

1. Reveal a green maneuver while having no stress

2. I have revealed a green so BB-8 barrel

3. Decide to Stay on target - turn to another maneuver and treat as red

4. Gain stress

and with EI if I actually can do this BB-8 barrel, lets say I pull EI action after barrel so I get stressed. I REVEALed a green and then decide to turn it to red with Stay on target. Do I actually get to do my maneuver and get double stressed or each use of Stay on target while stressed ends in giving your dial to an opponent?

Both card are labeled "When you reveal a maneuver", so you can play car in the way you liked as they are trigger at the same time.

Yep, you decide the timing so there's no reason you can't do them in that order.

Rulebook (SWX36, SW:FA Rulebook), page 4: "A stressed ship cannot execute red maneuvers [...]"

If you reveal a green maneuver, BB-8 for a barrel roll, and then PTL or EI for a second action and thus receive a stress token you may not resolve Stay on Target as you must "Treat your maneuver as a red maneuver." (FAQ v4.0 updated: 12-21-15)

You are, by the Rules as Written, protected from having your opponent choose another legal maneuver to execute if you try to trigger events in this way. The same rule that states you cannot execute red maneuvers while stressed also states: "If a stressed ship reveals a red maneuver, the opposing player chooses any non-red maneuver on that ship's dial for that ship to execute." If we roll back time you revealed a green maneuver, ergo the opposing player may not choose any non-red maneuver on your ship's dial for it to execute.

And here is where it gets a little sticky: you cannot execute the now-red maneuver you've attempted to take with Stay on Target. As the start of the Stay on Target card reads: "When you reveal a maneuver, you may rotate your dial to another maneuver with the same speed." You have now assigned your ship an "illegal" maneuver and I suspect you will find most TOs will rule your opponent may set your dial to any non-red maneuver. That being said, I would not be surprised to see a ruling stating that you cannot trigger Stay on Target therefore you must go with the green maneuver you originally revealed.

And to clarify it in my mind before I go mad with combos, the SOT card ALWAYS takes precedence with what colour the new maneuver is, right? No matter what other abilities you have, if you use SOT to pick a move it is red,

Example I thought of that prompts this: If Ello Asty programmed in a 3 move, then SOT'd it to be a T-Roll. His ability to make T-Rolls white get's overruled by SOT, right?

And to clarify it in my mind before I go mad with combos, the SOT card ALWAYS takes precedence with what colour the new maneuver is, right?

Yes. Anytime two or more effects changes the difficulty of a maneuver the more difficult one takes precedence.

Yeah, was 99% sure.

Still, means SOT now has a good droid to pair with it: The Targeting Astromech. Always change your maneuver, always get an action.

You are, by the Rules as Written, protected from having your opponent choose another legal maneuver to execute if you try to trigger events in this way. The same rule that states you cannot execute red maneuvers while stressed also states: "If a stressed ship reveals a red maneuver, the opposing player chooses any non-red maneuver on that ship's dial for that ship to execute." If we roll back time you revealed a green maneuver, ergo the opposing player may not choose any non-red maneuver on your ship's dial for it to execute.

All of the current rulings regarding revealing a maneuver treat any change to that maneuver during the "When you reveal" timing window as counting as a revealed maneuver.

You are, by the Rules as Written, protected from having your opponent choose another legal maneuver to execute if you try to trigger events in this way. The same rule that states you cannot execute red maneuvers while stressed also states: "If a stressed ship reveals a red maneuver, the opposing player chooses any non-red maneuver on that ship's dial for that ship to execute." If we roll back time you revealed a green maneuver, ergo the opposing player may not choose any non-red maneuver on your ship's dial for it to execute.

All of the current rulings regarding revealing a maneuver treat any change to that maneuver during the "When you reveal" timing window as counting as a revealed maneuver.

This is good to know. Do you have a page reference from the applicable FFG material?

You are, by the Rules as Written, protected from having your opponent choose another legal maneuver to execute if you try to trigger events in this way. The same rule that states you cannot execute red maneuvers while stressed also states: "If a stressed ship reveals a red maneuver, the opposing player chooses any non-red maneuver on that ship's dial for that ship to execute." If we roll back time you revealed a green maneuver, ergo the opposing player may not choose any non-red maneuver on your ship's dial for it to execute.

All of the current rulings regarding revealing a maneuver treat any change to that maneuver during the "When you reveal" timing window as counting as a revealed maneuver.

This is good to know. Do you have a page reference from the applicable FFG material?

This is one that kind of requires reading between some lines. Look at the FAQ entries for Navigator, Boba Fett (Imperial). They mention being able to work in combination.

If the orignal move you revealed stayed your revealed maneuver the abilities wouldn't work together. Both cards would end up modifying what the orinignally revealed move had been with which ever one you triggered second being the only one that really counted. If Fettigator revealed a 1 bank to the left, he could use Fett's ability to change it to the right and when Navigator was used he'd change the original 1 bank to the left to a 2 or a 3 bank to the left.

Everything works as expected if you treat any changes during that "When" window as being the manuever thatcwas revealed.