If I focus a unit to strike during an engagement and am then able to remove the token after the strike can that unit strike again during the same engagement ?

to add a shield to a unit with shielding do I have to declare them an attacker for an engagement?

as the Lightside player if the balance of the force token is with the Lightside do I automatically deal one damage to the objective of my choice?

If I have admiral ackbar in my had at the beginning of an engagement after attackers and defenders are declared can I use his action to play him for free? Or do I play his cost?

1) Yes,Rules page 20

Players continue to
alternate, focusing a unit and resolving its strike, until no
participating ready unit remains.

2) As per the rules page 25:

shielding
When a card with the “Shielding” keyword is declared
as either attacker or defender , its controller may
immediately place one shield token on any friendly
unshielded participating unit, or to the engaged objective
card, if unshielded.

3) Rules page 12

LS player is the active player
If the Balance of the Force is with the light
side (i.e., the token shows the LS faceup), the
LS player may deal one damage to any one DS
current objective.

4) Ackbar says: "play this unit from your hand"

Rules page 13:

Play describes an action where a player pays the resource
cost of a card in hand and transfers it into play (for a unit
card or an enhancement card) or resolves its effects and
then discards it (for an event card). (See “In Play and Out of
Play” on page 27).]

only thing to add is for "play" compare to "put into play" on human replica droid. Thes cost is not paid.