QI have this follower and one other Draconic Duelist card on my field. Since this follower has Assail from its passive ability, I declare an attack with it on a reserved enemy follower. If my opponent destroys the other Draconic Duelist card on my field at Quick timing, causing this follower to lose Assail, does the attack still go through?

AYes, the attack goes through on the original target.

QIf I have an Aftershock on my field, does it count toward conditions such as "while there is another Draconic Duelist card on your field" and "if there are at least 3 Draconic Duelist cards on your field"?

AYes, it does.