Leaky drop in tanks are usually caused by either the coil not seated properly and/or some sort of tolerance issue with the 0-rings and how the glass is seated. Smok coils are of intermittent quality so it could be your coils, I guess. Closing off airholes when filling, as well as closing them off they are going to float around in a satchel or something can help. I would check out some youtube videos on your device as well as break down the tanks to check if there are any missing or poorly seated or degraded 0-rings anywhere (usually you can find an assembly diagram if you aren't sure.
To be honest, my drop ins never seem to leak and certainly not in the kind of catastrophic fashion you describe. I don't usually have leaks and if I do, it's going to be on a rebuildable tank atomizer. The Crown shouldn't be leaky.
You don't want to fill the tank 100%. For one, it won't vape very well because there will be no air bubble at the top, and also, yeah, the more you "force" liquid into a tank well the more likely you are to exceed the tank's "tolerance" because it's really not designed to be filled so full, the liquid is squeezing harder on the tank, causing leaks. So if you are, stop doing that.
Best of luck I don't know exactly what's going on but those are the things I do know. Etc.
Anna