So the PPQ clock could get late and make the midi sequence drift in time, ...
Could or Does? Also, if you don't accumulate errors but always resync on the beat, some PPQ might be slightly off due to rounding, but the error will even out. No matter what you do, those errors will be present, the interesting part is to make them not matter. As an advanced topic, you could make sure the rounding errors get spread in a most even and balanced manner over the PPQ intervals so they're least perceptible. For this one can apply the algorithms designed by Bresenham in the 1960ies for line drawing on raster displays.
Korman