I did wonder if any of my problems have to do with some kind of editorial interference, but I have zero knowledge of the industry or the publishing process; I cannot begin to guess how likely it is. It also goes both ways: if they are not responsible for the changes, and if it was picked up thanks to the twitter version's quality, even if all changes are coming from the author, I'd perhaps except somebody to go 'hang on, this isn't what we signed up for'.
Incidentally, for my first wall of text I treated both works as monolithic, but it's notable that the humour I object to so much is completely absent in volume one, and starts up going strong in volume two. V1 has other tonal problems, issues with characterisation, and writing choices I consider inferior to the TV, but this change between the volumes is very noticeable, now that I think about it in those terms. No idea what it means, though.
I briefly mentioned it, but I do think making Sara the main viewpoint here makes a lot of sense on paper, I just think it wasn't executed well. I do absolutely see a tonal shift coming as the plot ramps up - there is foreshadowing for it already. However, the tone is not separate from other elements; it affects how we understand the characters and the story. Simply changing it to be more serious, without taking time to adjust the other aspects, is not going to be successful. Just as an example, if there are action scenes in the future where Natalia is suddenly depicted as a capable combatant, even if the scenes are played straight with no humour, it will not feel authentic without first giving her a training arc or something.