If a monster is forced to attack by "Amazoness Archers", and a replay occurs, then you cannot choose to attack with a different monster. You can choose to attack a different monster.
Previously Official Rulings
These TCG rulings were issued by Upper Deck Entertainment and have since been deemed unofficial by Konami. They were previously considered official and can still be correct, as long as they follow the official gameplay rules, or unless Konami has issued a ruling that says otherwise. References: 
The -500 ATK applies to each monster as long as that monster remains face-up on the field. You can activate multiple "Amazoness Archers" and the ATK reductions are cumulative (so -1000 ATK with 2 copies).
The effect of "Amazoness Archers" will make your opponent have to attack with all of his monsters if he has multiple monsters on his side of the field. If "Amazoness Archers" is in effect, and later a replay occurs, your opponent cannot choose not to attack.
Monsters that have already attacked cannot attack again. If "The Dark Door" is active, your opponent still can only attack with 1 monster. Monsters which cannot attack still cannot attack (Toon Monsters the turn they are Summoned, "Ultimate Obedient Fiend" under the wrong conditions, etc.) If "Diffusion Wave-Motion" is in effect, only the monster affected by "Diffusion Wave-Motion" can attack. The ATK reduction of "Amazoness Archers" is still applied in these cases where monsters cannot attack.
Diffusion Wave-Motion: If your opponent activates "Amazoness Archers" when your selected Level 7 Spellcaster-Type monster attacks, only the Level 7 Spellcaster-Type monster can attack because of "Diffusion Wave-Motion"'s effect, so your other monsters do not attack, but the -500 ATK effect of "Amazoness Archers" is still applied to all of your monsters.