OK, so the Paladin and Druid companions (I forget how to spells their names without mangling them, you can see how much I've used them so far) both have business in Twin Elms.

 

Are theirs the kind of quests that  will let me swap them into the party when I feel like taking care of them, or am I better off - for the sake of the story - having them in the party on the way to Twin Elms and throughout, because they're tied into the main plot?

 

Or is it something in between, like the way you don't need to have Grieving Mother in your party when you go into the Skaen temple, but it makes a big difference?

They have no impact on the main plot - just little side stuff. You can easily pop them in and out of the party.

You can return from Twin Elms if you want to go back to the stronghold and switch out NPCs.  It's only really late in the plot that reach the point of no return (and there is definitely a point of no return), though by the time you first go to Twin Elms, you are a lot closer to the end than the beginning, let's say.

 

Honestly though, I don't think that any of the pre-mades are critical to the main plot itself.  If they have a reason for going to Twin Elms, it's for their personal quests and not anything linked to the main story line.

It is easier to have the druid in the party, because he has several stops to make, but the paladin is just a one off conversation.  But you can do their tasks, leave, replace them with useful people and come back.  

@ the last two posts:

You know you can switch your party at inns right?

Thanks, everyone.