Found a good and cheaper vet who seemed to know her stuff. We talked about the wheezing, the weird cere, everything, but from the tests and examination, they seemed perfectly fine. Her conclusion was that Ludvig just has some weird hormones and was probably born with a weird throat, and that Melvin's issue with itching was caused by the molt. I made clear that it had lasted for a couple of months and had nothing to do with the molt, but well, he do appear healthy. She said it could be good to spray them with a solution with aloe vera, since it helps for the itching. She did say that we could try antibiotics just for the sake of it, but it could potentially do more harm than good. I agree, one should know what the issue is before prescribing antibiotics.
They are all back in the cage now and resting. The vet did acknowledge the cere probably being caused by a hormone imbalance issue, but she thinks it's just a quirk. I don't know, honestly. The most important thing is that they all seem healthy, and that the issues can be solved by time or will never be a problem at all. I'll of course run down there again as soon as possible if their health worsens. By now, they are happy and energetic, and apparently eating very well. Good news.