How do you resolve it if a ship not moved when it should have moved and after other ships have already moved? For example, Stele had a damaged cockpit crit and went down to PS 0. I had a gunboat at 4. Forgetting the crit, I moved the Gunboat. Then we remembered the crit.

If Stele wouldn't bump or block anything, the way I've played is to move him as soon as you remember. If there would be overlaps in positioning, the thing to do is to undo the move on the Gunboat (it isn't too hard, since the maneuver will probably still be on the dial), move Stele, have him perform an action* and then redo the Gunboat move (and perform an action if possible.)

* This is where being a good sport will come into play. If Stele's player says "well, I want to barrel roll now," and such a barrel roll would lead to the Gunboat getting blocked, well, then that starts to seem iffy. If they were genuinely not at fault for not being able to move in the proper sequence (like you moved the instant they said "my dials are set" and never gave them a chance to remember the crit), well, maybe you've just got some lumpy oatmeal. However, if there was enough time for them to remember and they forgot, or prompted you to move your ship, I don't think it'd be unfair to request that they strongly reconsider performing re-position actions with the extra knowledge they've gotten from the out-of-sequence reveal of the Gunboat dial. However, if they insist that this is what they'd intended all along (like, if they wanted to get position in relation to an asteroid ahead), the best thing to do as a good sport believe them, but maybe lay on a bit of a guilt trip.

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TL,DR. If the ships aren't close together, just do Stele second. If the ships are close enough together to matter, undo the gunboat move, next do Stele, and finally redo the gunboat. In either situation, both players should be mindful of the mistake and be good sports when selecting actions, as to not use the error to unfair advantage.

There's also the "you get one freebie" rule I personally use. If someone makes an obvious mistake, either by doing something out of order (like moving ships), missing a clear and important opportunity (forgetting to take a clear shot with their lowest pilot skill ship), or skipping a step that their build is clearly designed for (like using Fire Control System), I'll remind them once, then tell them that's their "one freebie."

In matters where the out-of-order resolution doesn't affect the game state, I usually just brush it off... for turn one, I don't care if you move all your ships in formation, even if they are all different pilot skills. But when it comes to repositioning ships en masse (moving a block of TIES as one, then barrel rolling them as a unit, is a pet peeve), or order of operation in the middle of a tight engagement, I'm a bit less lenient. Same thing if my opponent has 3 ships at, say, PS11, 10, and 9, which all outclass my pilots, but he accidentally activates them out of order in the combat round... you really gotta stick to pilot skill order, even if you WANT to have your PS11 shoot last.

RAW, IIRC, if you miss a move on a ship and move on to a higher initiative, that ship is supposed to skip its action phase. BUt I don't think I can provide a citation for that.

Mostly, it's possible just to wind back the clock and start over though and I don't think I've ever done anything else in practice.

Mandatory things like the order ships move in, rolling for console fire, etc are the responsibility of BOTH players to do properly. A player is never allowed to skip the activation by choice. If a ship of your opponent's that's been dropped to 0 gets forgotten then technically its on both of you and you still need to activate it.

If it doesn't matter positioning wise just move it when you realize. If there's potential issues with bumps or target lock range etc then you may need to rewind some other ships but unless someone has moved a whole TIE swarm or something this isn't usually a big deal.

Mistakes happen things get forgotten. If they are mandatory then both players need to make sure they happen regardless of who owns the ship. If there's a voluntary trigger (FCS, Rey focus, Decloak, etc) then that is solely on the controlling player to remember and it becomes a missed opportunity if they don't do it.