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.

QIf I have multiple copies of this follower on the field while my leader's defense is 5, will each copy's end phase ability be able to take effect?

ANo, they won't. Although each copy's end phase ability will be able to trigger, the effect of the first copy's ability will change your leader's defense to 10, preventing each subsequent copy's ability from taking effect. Therefore, your leader will only gain 5 defense, and you will only be able to draw 1 card.

QIf I have this follower and Lucifer on my field while my leader's defense is 5, will both followers' end phase abilities be able to take effect?

AYes, they will. However, although this follower's ability will trigger at the start of your end phase, its effect is based on your leader's defense when the ability resolves. Therefore, in order for both abilities to take effect, you must resolve this follower's ability before resolving Lucifer's ability.