Any 15 degree rotations are likely to result in off-grid vertices (except for 90 degree rotations), but that isn't likely to cause much trouble. Just be aware of the fact that those vertices will be snapped to the nearest grid point at compile-time. For rough architecture brushes, it isn't very clean to work with so it's not really a desireable situation, but for smaller objects, func_details and such, it isn't something to worry about. For the rough architecture however, there are cleaner ways.
There are multiple ways in fact. You could, indeed, rotate, but that's the least elegant way. Better is the
shear method, which allows you to slide one of the sides away. And of course, vertex manipulation is possible, but it's a quirky tool if you're not familiar with it. There are a few rules to keep in mind, such as that faces must always be completely flag (e.g. a curved face is invalid). However, if you select vertices in the 2D view, with a normal brush, you'll always select both the top and bottom vertex, so moving them around won't usually cause trouble. Just don't go dragging individual vertices around. It's probably better to go for the yellow dots, the edges, those may be easier to work with for you, in this case.
I couldn't find any funny brushes in the map you provided bytheway. Are you sure you uploaded the right one? I saw an L-shaped corridor with a triangular hint brush placed inside, and an outside room with some combine barriers.
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