If I play an unit from my hand during an engagement with Jabba's action, is this unit considered as participating unit in this engagement ?
Other question with the same fact but with Vader Navy version, is this unit gained the blast icon ?
No, unless you use it during the action windown after attackers are declared and before defenders are declared. You could then declare it as a defender. The unit is just put into play and not put into play as a participating unit on your side.
Yes, Vader gains the blast damage icon.