Hi Paul,
In answer to your questions.
You should use the “CV” filter where you take unfiltered photometry and calibrate using V filter comparison star magnitudes. This filter is standard in the BAA and AAVSO and is there exactly for this purpose.
It is common to add images if the SNR is very low, or to take longer individual subs.
It appears the AAVSO have a sequence for AT 2019xim. It is fine and common practice to quote AAVSO sequences when submitting to the BAA Photometry database when a BAAVSS sequence is not available or if it is not appropriate for digital photometry. They have far greater resources available to produce sequences. Just quote the chart reference, e.g. AAVSO X25107PN or X25107PN.
Best wishes,
Andy