Hello, can i use a Mercenary skirmish upgrade (Last Resort) on BT-1 if i have him in my Mercenary list using elite Jawa?

Yes.

Yes. You can only include skirmish upgrades of your own affiliation. At the start of a mission each skirmish upgrade attachment must be assigned to a deployment card in your army, max one per deployment card, regardless of affiliation (but satisfying other requirements like "Darth Vader only").

Thank you!!!