For source strengths below a few counts per second, the flux within the PSF is small enough that the probability of interference between the pixel patterns of individual photons is small. In this case there is no error in either flux or photon energy determination, and pile-up is negligible. As the flux increases the patterns from successive photons detected in the 2.6 s integration time start to interfere, and pile-up becomes significant. Since flux removal is the main effect it is moderately safe to form the spectrum from the remaining flux even though the centre of the PSF is missing. In general it is safe to err towards imaging mode or large windows, unless the source is very bright.
Timing is related to the CCD readout and through the electronics to the spacecraft clock.
The precision of timing is limited to the effective integration time. The accuracy is achieved via the spacecraft clock to UTC, and for a given sample there is a s jitter caused by the fine time code sampling of this clock.