I have some questions about this set of images. I've plotted the start times below.
There are four groups that last about 45 minutes, each followed by a break of either 45 minutes or 90+45 minutes, is this related to the HST's orbit?
I notice a slow rotation of the star tracks over the duration of the roughly seven hours of data, it looks like roughly 20 degrees, (see GIF or the individual images) and I can't figure that out at all. Is it a rotation of the HST around its own axis?
Individual preview images (example) appear to be rhomboidal rather than square, why?
TL;DR version, Yes, yes, distortion. In more detail:
HST's orbit is approx. 90 minutes so this will cause the breaks. Some more information is given in the Orbital Constraints section of the Cycle 27 guide for proposers.
Since HST is having to move to track the moving comet to keep it in the (small) field of view of the instruments, it will likely need to roll the spacecraft. This is to keep suitable guide stars in the Fine Guidance Sensors, to keep the Sun on the solar panels for power and to keep the pointing constraints of not looking to close to the Sun, Moon or Earth satisfied. These are discussed in more detail in the Orientation and Roll constraints section of the primer/guide for proposers mentioned earlier.