I think Sarevock’s was done well but Viconia’s was not ... *snip* ... But a true alignment change needs to be supported by good plot points not by romancing, etc.
Definitely agree on both these points, and particularly that companion character arcs shouldn't be dependent on romances. Btw, when it comes to BG2, Anomen's possible turn to CN (I think?) was pretty shaky too, but perhaps I just think that because I'm not a fan of Anomen in general.
I actually wouldn't call the sort of companion malleability I often enjoy either redemption or corruption arcs, as I think it works best when everything is there in the character from the start and it's just tipping the scales. And I reckon all the companions we have so far have shown that potential already. For example, will Shadowheart's newfound sympathy for tieflings lead her to reject some of Shar's more unforgiving teachings and lean into the sort of positive spin that she gives the PC when she gets her memories back, or will she come to despise herself for her weakness and become more zealous than ever (assuming she doesn't discover she's not a Sharran at all)? Will Gale find a way to rid himself of the Netherese orb and resume life with a new sense of humility, or will he find a way to harness its power and make Mystra sorry she rejected him? Can Wyll free himself from Mizora and start to become the hero he'd dreamt of, or will he compromise himself beyond redemption as he struggles to extricate himself from the bargain? Will Astarion seize the power he's been granted and overthrow Cazador and take his place, becoming an even worse monster, or will he destroy his former master but embrace his freedom and walk away from vampire power struggles (while still being a selfish git, of course)? And might Lae'zel find a way to prove herself and become Vlakith's champion, or will her experiences among the infidels shake her faith and ... probably leave her bitter, angry and lost? It feels to me as though Larian have sowed the seeds of these possible character arcs, as well as lots of other ways the companions' stories can go, and I really hope that there's not only one answer for each but that exactly how things play out can be influenced by the story we create.
Whereas I can't see Minsc being anything other than Minsc (well, I could but I wouldn't want to!) and Jaheira has always struck me as pretty inflexible. And they're both plenty old enough to be stuck in their ways, so I can't really imagine that they have different character arcs, even though I'm sure they'll have different stuff happen to them depending on our choices. But perhaps that's a lack of imagination on my part and Larian will surprise me!
EDIT: I just spotted Ragnarok's mention of Alistair in DA:O and remembered that he has one the most egregious examples of a poor character arc I've come across, whereby whether or not he becomes "hardened" and therefore certain endings are available for his story comes down to a single dialogue choice that's not obviously important. Ugh!
