Speaking personally and with my visual hat on, there is no way a visual observer can compensate for the background galaxy. I’m sure there lies somewhere an obscure equation to allow for this, but I don’t know of it. I have always been sceptical about applying corrections to visual observations – even for red bias in personal equations – as there are just too many factors to take into consideration. Far better to leave well alone.
I can see some unusual behaviour in the light curve in mv compared to V as the SN has faded, and unusually the fading trend is slightly different too, which I find very interesting. In most cases the trend remains similar whilst there are differences in measured values.
Your experiments in removing the galaxy from the image are fascinating, but it doesn’t surprise me that your V measure changes. I hope you’ll keep us informed as to whether you can improve the difference between the two measures. In the meantime I would urge visual observers who are observing this SN (and lets hope that as it fades visual observers keep at it) not to worry about the galaxy at all, and put all their efforts in to obtaining the best visual measure they can get.