Welcome back!
That’s a lot of points. 
Confusion is the expected and normal state at this point! We are all confused.
Things have changed a lot, many of them in the last couple of months and even weeks, so you will be reading and watching a lot of stuff that is out of date. wowhead.com and icy-veins.com should be pretty current, but check the dates on anything else.
Heirlooms are no longer better than regular gear. Blizzard nuked them from orbit. They don’t even give XP bonuses anymore. They are handy because they never fall behind your level, but they are nowhere near required.
Scaling is … bugged? Not exactly in specific ways, but yeah, it produces inconsisitent results. Scaling makes performance all over the place, especially during prepatch. A level X may apparently smoke a level Y on the DPS meters just because of scaling irregularities. Pay no attention to what your DPS meter says when people in a party are not the same level - especially when comparing yourself against level 50s.
Azerite, Essences and Corruptions are gone, or about to be. All of BfA’s systems are being obsoleted for Shadowlands.
At some point in prepatch, quests will appear that will kit you out in Normal raid level drops. Not that that really matters anyhow, since you will also replace those in a few hours when levelling in Shadowlands.
Nothing below level 50 matters! Gear is all over the place, scaling is weird. Don’t sweat any of it.
It doesn’t matter which expansion you level in. The gear you get from dungeons and quests will be according to your character level, not the expansion. A level 33 will always get level 33 gear, no matter what expansion. Talk to Chromie near the Stormwind or Orgrimmar Embassy and pick whatever expansion you want to see.
Levelling has been vastly speeded up. You last played in Cata? So send one character to Mists, one to Warlords, and so on. Take the opportunity to see what you missed!
EVERYTHING below 50 is pointless, except to enjoy it! The artefact weapons may be pointless (well, Ashbringer is pretty pointy :P) at level 50, but the quests to get them are very well done, and worth seeing. Besides, they make for great transmog.
Chromie doesn’t offer BfA as an option, AFAIK? I tried on an alt, and she didn’t have it for me. Instead there was a quest at Grommash Hold - I imagine that would be by the Hero’s Board in Stormwind - to go to BfA. But maybe that’s just another one of those changes that we haven’t caught up with yet.
From a Wowhead comment:
Chromie’s ability to send you back to the BFA timeline after choosing a previous expansion appears to be bugged in pre-patch.
After trying out Outlands, I have yet to find any way to re-start the BFA questline. (And yes, I’ve tried all of the obvious things. Including literally going to Boralus.) - When asking Chromie to send me to the current timeline, I appear to end up in the MoP timeline, with all the quests in the Stormwind Keep wanting to start the Pandaria intro.
All the other expansions seem available tho, via Chromie.
Edit Blizzard have confirmed there is a bug with this questline. It appears new characters that go through Exile’s Reach get the starting quest, but other’s may not.
HOW you get to level 50 doesn’t matter at all for game purposes. You will hit 50 in much the same gear whichever expansion(s) you choose and whether you go quests aor dungeons.
Just keep killing stuff, you eventually get to 50, the prepatch quests will arrive, and you will have your characters ready for the launch!
