[Rules] Leader activated several times in a turn

[Shad]

Ex 2: Leader x activates and activate all the units in his hex and adjacent hexes. All the units move BUT the leader. He stays in his hex and so he is not marked in any way. Later other units move in the Leader x hex and adjacent to him. May he be activated again to activate all the newly arrived units to move or fire normally?

No.

An activation must either be to MOVE or to FIRE and must be declared before doing anything further, per Rule 3.13.

So in the example you give your leader declares a MOVE activation but does not move. Whether he moves or not is irrelevant. (sometimes you will declare a FIRE activation with multiple units and the final unit will no longer have a target to shoot at, too bad so sad!) He still receives his MOVED/FIRED marker per Rule 3.13.