I have one question. I thought it will be possible to have a custom party without tricks.
But, that would mean either:
-- we're losing custom characters (if chosing companions)
-- custom characters remain undismissable
First of all, whenever we loose companions is not 100% certain. While other say that Larian wasn't clear enough I think that their statement in
Q&A is rather clear as to their intentions:
Will companions be interchangeable during long rest?
Yes, at the start of your adventure your recruited companions will be at camp when not in the adventuring party, and can be swapped in and out at camp. Just like friends in real life! After the first act however you are going to have to commit, also just like in real life.
We can only guess what that could mean exactly, or if something changes in the last two years. I am pretty sure they said there will be a way to have custom party to start with, but I also don't know what what would mean exactly - would Larian allow us to just create 4 coop player NPC without multile game instances running trick? Or would they give us an option to create merceneires (aka, D:OS2) during character creation, that can be dismissed and killed?
> We gain Minsk and Karlach later, so confirmed set of 3 + 2 companions?
I don't think we know when companions not in EA will be recruitable, no? I will be surprised if Karlach won't be origin/recruitable in act1. Yes, she currently dashes away and says: "see you in Baldur's Gate" but it feels like she is meant to join us like other companions - she just doesn't do so in Early Access.
After Game Awards trailer, I assume Jaheira and Minsk would be additional non-origin act2 companions, but Jaheira's (?) line from most recent trailer suggests her presence at/before Moonlight tower. Will Minsc be there captured as well? Would they be recruitable? Or would they just be NPCs for end of act1 and depending on our actions recruitable companions in act2?

Possible playthrough problems: reliance on Astarion when not playing rogue (confident). Chosing between Wyll and Gale, strongly favors Gale in general (correct me if I'm wrong). Shadowheart -- possibly -- comes whether you want or not if she still has the artefact. This costs you a slot.
5e doesn't require rogue - Astarion has been killed in most of my EA playthroughts and I didn't miss skillset at the slighterst. Anyone can openlocks/spot and disarm traps now. With multiclassing would allow you to make rogue hybrid from other companions if you wish to do so. I can't speak for wider companion preferences, and I can invision party compositions where both Wyll and Gale could co-exists. Larian already made it so, that the weapon gets tranfered to player if Shadowheart isn't around. I would guess the same would happen later on, if player would decide not to travel with Shadowheart anymore.
Edit.
Since that can't happen, they must die on their way there, right? Either that, or the party gets split against their will. There's no explanation to justify leaving them behind. It's death, mind control, or kidnapping. Ugh. Getting lost near Moonrise towers doesn't do wonders for your survival.
Here is my theory:
In the cutscene where we almost come under control of the Absolute it is the weapon that saves us from falling under their control. If Shadowheart isn't around, the articact gets tranfered to our Avatar. My guess is that at some point during act1finale/Moonrise tower, the artefact will protect our currently selected party, but everyone back at the camp will come under the influence of the Absolute. That way that can come back as antagonist - something Larian hinted at and half-hazardly did in D:OS2. I assume they have an idea that they didn't manage to fully realise in D:OS2, and they are giving it another shot in BG3.