RP can't exist in a static world. Why should I try to get into the head of my Tauren Druid when the evil in Wailing Caverns is just going to respawn after I leave? RP requires a sense of accomplishment beyond experience points and shiny.
It is a matter of selectively tweaking the story and setting to fit your character. This is not for the benefit of others, ultimately it is for your benefit.
One way is to say you woke Naralax and now the Barrens can recover properly but of course it will be slower than had he remained dreaming but uncorrupted. If you decide to go back to WC then ignore it as far as your story goes, think of it as cleaning out the remnants of the Fang, or treat it as a flashback to when you saved him previously.
The thing about being a serious roleplayer is that there are always
going to be events that pull you out of the experience, whether it is people talking out-of-character, stupid names, repeating content or what have you. You have to learn how to take those and either twist them to fit, make them work like an alternative way, or ignore them outright.
I will use whichever makes the most sense to me at the time, although my favorite is to tackle them in a tongue-in-cheek way that has my character taking them seriously but shows I'm having a sense of humor about it. Given the right situation it also flusters non-RPers, which I take as a bonus.