Maybe you misunderstood my concerns. Drilling the tank is fine, no problem there. All the links you provided were using a sump, again no problem there. That's a standard setup, works great.
The concern is using a canister filter, which is completely different in design when compared to a sump and return pump. Canisters weren't made to withstand the pressure of 18 inches of water. Think about the difference of running filter pipes over the wall vs having them under the aquarium. With standard over the wall installations the canister works on a siphon, there really is no pressure being exerted on the lines. When you move those lines to the bottom of the aquarium you now have water pressure fighting against both your inlet and outlet. I'm not confident that the canister seal will handle that for an extended period of time..