Two walls have been rebuilt due to moisture damage. The builders tore down the old walls and the polystyrene insulation. New mineral wool insulation was installed, and plasterboard was put up over it. We filled the seams with fine filler and only read afterward that paper tape should have been used over the seams first. What could happen now? Will the filler crack or not? The plasterboards are very uneven, meaning they bulge out and one board can be 3-4 mm higher at the seam, meaning you have to use a lot of filler so that it's not visible. In the corners, the builders haven't done anything, there are quite large gaps in both the outer and inner corners, which we now understand should have something in them. What happens if you don't have corner protection on the outer corner? Can't you use latex caulk there too and then wallpaper?
The seams are almost certainly going to crack, unfortunately. Also, it shouldn't be fine filler but medium. If the wall is uneven, you fix it with broad spackling, and for the corners, you use corner protectors so they're not so sensitive to impacts. You use latex in the inner corners.