So, if I understand it correctly, I can do the following.
1- A first unit sucessfuly score a markerlight on a vehicle.
2- A second unit call in for a seeker missile on the vehicle and then fire all it's weapons on another unit (ork boys for example), while the shas'ui with it's target lock shoot it's markerlight to another target.
Am I right ?
No idea what you're trying to say here. Here is how it works.
Using your example of 12 fire warriors, one of whom is Shas'ui and has a markerlight. They fire 11 pulse rifle shots at an enemy unit and the Shas'ui fires his markerlight. All your FW's must fire at the same target (unless the Shas'ui has a target lock, ignore the target lock for now). Assuming the markerlight hit and the enemy unit still lives, you now place a markerlight token next to the enemy unit to signify that that unit has one markerlight "marker". This can be a dice, coin, or whatever you want. That marker stays there until the end of your shooting phase or until you use it. The fire warriors are now finished with their shooting phase action and are furthermore ignored.
Now, later in your shooting phase, you decide to use the markerlight token. You remove it and gain any of the benefits listed in the codex. If you choose to fire a seeker missile from a vehicle equipped with one, then you do so and roll as if it were BS 5. If it hits, you resolve it as normal. Firing this seeker does not count as the vehicle firing - in fact the vehicle in question did not act at all yet, or it may have already acted - it does not matter. The seeker is simply carried by the vehicle and basically fires itself using its artificial intelligence and it targets the unit marked by the markerlight.
Now, if the Shas'ui has a Target Lock, he can attempt to fire his markerlight at a different target than the rest of the fire warriors if and only if you succeed at a successful leadership test. If successful, then you can target one unit (perhaps a vehicle) with the markerlight and a different unit (perhaps some ork boyz) with the pulse rifles. If the markerlight hits, you resolve as above.
The markerlight marks a target and a seeker missile can hit the targeted unit. The vehicle that had the seeker on it can do whatever it wants to.
Clear?
.