1. Which difficulty should I choose? Honestly, 'Explorer' sounds quite inviting with no stress after work, but I don't want my gear and upgrades to mean nothing. Should I just stick out 'balanced'?
Balanced should be fine. While not restricting you (Explorer prevents multiclassing).
If you find it too difficult, you can always change to Explorer mid-playthrough.
2. Should I invite everyone to camp and do their stories?
Yes. For the most part, you don't even need to swap party members in to progress their personal quests, you can simply talk to them at camp. As a new player you likely won't even know WHEN to swap them in anyway.
It's only for their Act 3 quests as well as some more obvious situations (Like going to the place where Lae'zel keeps telling you to go) where their presence makes a bigger deal.
3. Any extra tips and suggestions would be greatly appreciated!
- Talk to everyone. Learn Speak with Animals and talk to all the animals too.
- Explore. There are plenty of hidden trinkets and areas. Some of which will provide alternate routes to complete quests.
- Save frequently. You might run into unexpected situations you're not prepared for. So it's nice to have options to reload a recent save rather than lose a lot of time you spent exploring (While the game does have autosaves, they're not always the best)
- If you're using Shadowheart in your party, have her talk to Withers in camp to respec her as soon as possible. Change her starting stats so she either has good Strength or good Dexterity and give her a suitable weapon (I.e. Finesse weapon if you chose Dexterity) so she can actually hit things since her initial stats are pretty bad.
- Don't be afraid to utilize Withers to change your builds. If a choice you made is not panning out like you hoped, you can always respec and change it. This works for everything, from feats, spells, subclasses, classes, stats, you can respec any character almost completely (Only Background and Race cannot be respecced)