I reckon if your chainwhip technique is correct then riding the bike won't be making the cog any tighter. Reason I say this is that I can fit a cog with a chainwhip and no lockring for a 100kg+ sprinter with 2k+ watts in their legs and they can skip skid and trackstand on that without it coming loose.
I know there's a difference between that and whip skidding down the hipster spice route but when track standing they are pretty much riding backwards up a gradient.
They can also do standing starts and sprint efforts ie put all of that 2k+ watts into the drivetrain and nothing strips and the cog comes off easily with a chainwhip.
Given that you've had repeated failures I'd have said the fault lay with either your technique or your equipment (cog, lockring, tools). You say you've had 'an actual mechanic' fit your stuff and its happened though so that maybe removes your technique from the equation but, fitting a cog and lockring isn't something that I'd ever entrust to anyone else and I know/work with some pretty good mechs.