Cannot get there with alts even with explorer and loremaster on my main from those areas…there is literally 0 way to get there without play trough campaing and in case of SL there is no way…because in oribos after start main quest is no npcs and no quests.
Both Nazjatar and Mechagon is accessible to alts without having to do the campaign - you may have to do some of the intro quests for the areas themselves, but you can get there without the campaign
You can get to Nazjatar by taking the boat near the island from where Orc’s & Trolls have their starting zone. The small islands at the bottom of Durotar.
If you’ve unlocked all the flightpaths in Shadowlands on one toon, then your alts should be able to fly to those zones.
You cant skip to Maw or Zereth Mortis though, you need to complete the campaign for those zones.
As for Kul tiras & Mechagon I’m not really sure.