While cleaning the soundboard from dust with a Swiffer, I marked it very slightly with the rod of the cloth; two scratches appeared that are only visible in backlight. Can something be done to hide them or do I have to accept them now?
Unfortunately, there is nothing you can do. To clean the boards, you need to use something very flexible and very protected. If you reach a scratch, you can try on a small part, using a very fine brush to fill the scratch with a clear gloss (like clear nail polish... but more diluted, otherwise it creates thickness)
It depends on the finish; if it is polyurethane, there isn't much you can do. If instead it is board lacquer, you can quickly rub over it with an alcohol-only swab (but don't do it yourself because, given the question, you are not capable of it) and the scratch disappears.