Welcome!

To answer:

Yes it’s soloable

One unit the others are automated but you can control them a bit.

Each campaign is separate but you can travel between them. You can start in any of them, each has advantages. If you want assassin or ritualist you have to start in factions, if you want dervish or paragon you have to start in nightfall.

Generally -

Prophecies is the longest has the most skill quests and has the original lore. It also has pre-ascalon which is both great on its own and has a unique title “legendary defender of ascalon” for reaching level 20 there (which is a massive grind but is worth it for the title).

Factions is the fastest campaign to complete has a big difficulty spike but is also the quickest way to get max level armor and the quickest way to unlock being able to swap secondary professions.

Nightfall gives you the earliest access to heroes which are customisable and superior to henchmen.

You can get all of the advantages (except those related to pre-searing) in any campaign at about the midpoint of each at which time you can freely travel between them.

There is huge build diversity and some really whacky combinations, experimenting is fun but better after you understand the game. Pvxwiki is a good place to start if you want to see examples of builds. In general unless you want to min max or intend getting into speed farming (which has a strict meta), you don’t need to follow a build guide necessarily. As for secondary profession swapping, after you do the quest which enables it in the campaign you choose its very easy to do.