I have had no galahs but I have a close friend that has had one for years, a female called Rosabella. They are beautiful animals, no doubt about it! But they are cockatoos and that means EXTREMELY needy (you need to be home during the day and spend many, many hours with them) and very loud (although galahs are not the worst, the moluccans are). Even when they are happy with their life, they will still make flock calls at feeding time (mostly in the evening) and they will chew and chew and chew and chew EVERYTHING they can find so they either require a dedicated birdroom where they can chew everything they feel like (and that means walls, doors, windows, baseboards, moldings, etc - so everything needs to be protected by something they can chew). My Linus Too (LSC2) just pulled about 3 ft off the baseboard off the wall and chewed a hole in the wall. All the wood (baseboard, moldings, etc) in the birdroom where he lives are covered with boards of untreated pine which my husband replaces once he chews through them so he did not only pulled the 'protective' board and 'put it aside', he went back and pulled the actual baseboard off the wall so he could get behind it and chew a hole in the wall itself. They are VERY destructive animals and keeping them in a cage is no solution because they pluck and scream like maniacs if you do. You simply need to learn to live with it and prevent, as best you can, the complete demolition of your home
The other thing is that they are intensely jealous of their chosen human. My friend had -and still has- a lot of trouble with this because even though her children are all grown now, she flies out to attack them as soon as she sees them (she knows they are competition for my friend's love) and males are worse than females.
Can they sever a finger? Yes, they can. They can also bite half a lip off as well as half an eyebrow or half an ear. They have big and VERY powerful beaks! Now, the toos I've had (umbrellas, citron, lesser sulfur) never really bit me that hard but I have ended up with holes in my arms and bruises from them just hanging on to me (they are large and heavy and have strong claws) and, of course, wounds from bites (because even a little bite is a big bite when it comes from a large bird).
Cockatoos don't have 'terrible twos', they have terrible fours and fives - which is when they become sexually mature but I've never found that a bird going through puberty is that much more difficult than any other bird. And yes, they do change as they get older. All animals do. As to their temperament.... well, I don't think that any species is 'mercurial' to tell you the truth. They do have breeding behaviors that make things a bit more difficult for their keepers during the season but as long as they get the right kind of diet, a strict solar schedule, good quality full spectrum lights and good care (flight, many hours of out-of-cage, company all the time, several hours of one-on-one, etc) they do not become overly-hormonal so their breeding seasons are not that very long and the behaviors are not acute. It's bad care that makes birds mercurial, as you call it.