You are confusing the wildlife act with the firearms act. Dont worry, a lot of people do it, and on occasion I do too.
The wildlife act is written to be 3 rounds in the gun. So one in the chamber, 2 in the tube. It does not say anything about shell size. if you are using mini shells, you are supposed to be limited to 2 minishells in the tube. But what mad lad would do that? So the CO's will measure your tube for 2.75" shells.
I understand your view of the matter, but you are confusing a mag capacity from the firearms act with the wildlife act. I know you are talking hypothetical, im not saying your upto no good.
If it helps, My 3.5" shotguns are plugged so they can only fit 2x 3.5" shells and fits like 2.5x 2 3/4" shells (not 3x). I hope this helps.
I agree with you and ButcherBill. I am not confusing anything. My assertion was based on no specified size in the Fish and Wildlife act and the wording of the firearms act where it says no more than 5 of the shell the gun is made for.
If a gun is a 3.5", a tube length that is 8.5" long meets the "no more than 2" if using the size of shell the gun is made for. This should be a given. Even if someone is using shorter shells, provided they don't have more than 3 in the gun total they should be fine. It would be unreasonable to expect a permanently plugged (as required by law) shotgun to be altered each time you switch shell length.