YES!
Your last reply worked for the NPCs with single quests. So I shoved on the line that was working for the NPCs with multiple quests before:
/run SelectAvailableQuest(1)
…and it works perfectly now!
Thanks so much, I never would have figured it out myself. Here is the macro in its entirety:
/run C_GossipInfo.SelectAvailableQuest(C_GossipInfo.GetAvailableQuests()[1]["questID"])
/run SelectAvailableQuest(1)
/run AcceptQuest()
/run CompleteQuest(1)
/run GetQuestReward(1)
Thanks again!