You need labels, ones from something that she agree's to but has no idea what is in the tin, then just swap the labels over to the good food. However I will say, as unpopular this may be, under her roof, her rules. Easy for me to say as I moved out at 16. From what I can see you have no good options
but we are here for
and if you need additional help for advise then just ask you mum specifically what you need in print prove that merricks is better than Pedigre power chow (or whatever they call it) and there will be people able to direct you straight to where you need to go rather than spending time looking for stuff, being on canned limits the choices (costco kirkland premium was what I was thinking) if finances come into play. Good luck.