What are Delves and how do I start them as I understand they are good for gearing up new characters.
The other question is: can I go back to the timkewaked previous campaigns once I hit 80?
First you start the TWW campaign at level 68. At some point it will lead you into a delve. Delves have 11 tiers. While leveling, you can only do up to tier 3.
Once you hit 80 and finish the campaign on at least once character, it unlocks the ability to do higher tiers. The highest rewards are from tier 8. Tiers 9-11 are challenge tiers.
To get gear, you need restored coffer keys which you get from the first 4 world events you do each week (Awakening the Machine, Theater Troop, Spreading the Light, spider weekly, special assignments). You also get a few from renown and sometimes world quests. Every day, one delve in each zone will be Bountiful. If you run a bountiful tier 8 delve, you use one of your keys to open a chest at the end that will have 603 gear. If you do eight tier 8 delves, you’ll be able to choose one of three 616 items in the vault the next week.
Delves are fantastic at gearing up new characters, doubly so if you mix in raiding and/or m+ while you do them.
I’m not sure about the timewalking campaign. I’m pretty sure you get kicked out of chromi time and can’t go back but it’s been a while since I leveled a new character.
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Althalas has given you a great answer about Delves. Delves aren’t wonderful for gear before you hit 80. but working your way up to Level 8 once you hit 80 guarantees you a reliable source for good early max-level gear.
I’ll throw in this guide to Delves
and this commentary on the easiest Delves
https://gamerant.com/wow-the-war-within-easiest-delves-best-bountiful-delve-tier-list-tww/
Brann is a HUGE help in the higher tiers of Delves. Getting him levelled up by completing Delves and by picking up small items on the floor that increase Brann’s XP.
I recommend the Healer/Taunt build while you are gearing up. It’ll save your life many times. 
No.
If you want to do a previous expansion, say Northrend/Wrath, I recommend making another character to do it. Then you’ll not only have done it, but you will have another alt at 70! 
You can go back on your level 80 and do the quests, but it will be no fun at all, since at 80 you will effectively be a God, and the mobs will pretty much die of fright when you look at them. You will get no XP, no useful gear.
Thank you, that’s very useful info