Ok, so the shiny mew I have was checked by the anistar city memory checker. It says that the mew "has a good memory, but doesn't seem to remember" which means it's definitely from an older gen. Also, I checked it's profile and it does not have the kalos pentagon, further backing up the fact that it was transfered from an older gen (gen3). It has the JAP tag, is in japanese and is from hoenn, matching the southern island mew from that time. It even only has one good IV, meaning it was not RNG tampered with, even further backing up the fact that it is legitimate because the RNG wasn't known back then. Should I assume my shiny mew is legit? Because if so then this is groundbreaking news. If you suspect it is, perhaps I can bring it to the right person to verify for us all?