QIf I have multiple cards with an "At the start of your end phase" ability triggering, in what order do I resolve them?

AYou can resolve them in any order. When you proceed to your end phase, all "At the start of your end phase" abilities are triggered and become pending. Then, you can choose the order in which you resolve the pending abilities.

QAt the start of my end phase, if this follower's ability triggers and I summon a Mystic Artifact, when can my opponents play Quick cards or abilities?

AThey may play them after both this follower's and Mystic Artifact's abilities are resolved. The order the abilities are resolved is as follows:

1. At the start of your end phase, this follower's ability triggers and becomes pending.
2. You summon a Mystic Artifact token, resolving this follower's ability. Since Mystic Artifact has Ward, you may decide whether or not to engage it. Then, Mystic Artifact's Fanfare ability triggers and becomes pending.
3. You draw a card, resolving Mystic Artifact's Fanfare ability.
4. You may engage any followers with Ward on your field.
5. Opponents can now play Quick cards or abilities.