If a player has the "Code of Laws" Tech Card, are the abilities available only if he is playing as a Republic.
The ability on the Tech card is available. The ability of Republic is not available until you switch to that government and is only available while you remain a Republic.
Are the unused cards in the battle hand considered to be "surviving units"?
Those extra cards can still be played to new fronts and will count towards your total combat strength at the end of combat.
Is attack strength the same as health strength, such as when using the resource ability of Metalworking? So if my unit has a 1 strength value before using Metalworking, would the opposition require dealing 4 wounds to kill that unit?
Normally, attack strength equals health, but IIRC Metalworking only increases attack strength.
Hope this helps.