Wow, this is a long answer and I don’t know everything.
As for Chromie Time - all that does is allow you to play in any timeline you choose without leveling out of it. Normally, in most of the expansions would you level out and the quests would become trivial. Chromie allows you to get all the way to 50 while still getting experience like it was current content. Once you hit 50, you get a pop-up and booted out.
So in Draenor, when I hit 50 on my Chromie toon, the mobs were no longer 49-50 to me. Instead they were 35 and gave no experience and died if I gave them a stern look.
The chronological order happens in order of the expansions, but Warlords of Draenor is a little weird, because it’s a separate timeline/reality, and most of the characters in that expansion were around before the base game in our timeline/reality.
As far as Chromie time, I think if you stay in this current time and try to do past content, you’ll out-level zones very quickly. i.e. Westfall stops giving XP at 10, Duskwood stops giving XP at 20? So I think you have to do Chromie time and select the appropriate expansion in order to get the most out of each zone. I could be wrong though.
i think vanilla is gone, so the best order would be to play Classic WoW to 60, then switch to retail and go through starting with the Burning Crusade, Wrath, then Cata aka retail vanilla, then so on.
As others have said, the expansions go in chronological order but there are some oddities. With Cata, they revamped the old world so the questing in Kalimdor and the Eastern Kingdoms actually takes place after BC and Wrath. Plus, as has been mentioned, Warlords is an alternate universe expansion so you’ll run into doubles of characters you’ve already met.
It’s not required but the zone scaling is limited without it.
There is no order for the story. Each xpac is like it’s own game story wise. Very few things are mentioned from previous xpacs, events in Wrath are barely mentioned in Cata, likewise for the other xpacs.
Not really. Some of the zones lead into other naturally but there isn’t an overarching story like you get with the expansions.
The closest I can think of is to make an alt and have Chromie pop you into the Cata timeline. That’ll give you a breadcrumb quest to get going but, like I said, it’s not a single storyline.