Archers don't move so receive extra attack- can they split the attacks? If first target alliminated can they use second attack against new target?

Yes. They are allowed to perform a second attack, nothing is said about who they have to target with such an attack. So, you're allowed to attack twice the same unit, or two different units, as you like it

Yes, confirmed. It is usually safe to assume with FFG that if they don't tell you something is limited and it is not already a previous rule, then your good to go. FFG is very specific and good at telling you what you can/cannot do.