Is it possible for the attacker to loot the defender minerals if they won? Or the only way to gain is by conquer and tax only?
If you mean when someone else attacks you and wins, they take all your resources, no. Even if you get conquered you don't really "pay tax", even though your conquer will get taxation. So you don't lose any res at all, unless you got spy attacked, ofc... but that res goes to no one.