The first point you make is not about consistency, but putting basic gameplay mechanics into place.
To your point though: it's about putting 'consistency' above everything else and never has been. Consistency helps an awful lot though if there is no intuitive reason that things should be different. A bundle nade being stronger than a normal one? Makes an awful lot of sense intuitively. Elite infantry being better than mainline? A P4H being stronger than a M4A3? Same.
The same weapon doing a different amount of damage against similar units (=target tables)? Oof, in my eyes a far stretch in 95% of cases and there must be good balance reasons to do it like that. It is not intuitive that e.g. a 222 is more accurate vs a sniper than vs a normal model. This is pure balance.
Anyway, regarding the canister shot: As I tried to say in my first paragraph, it should be considered if an automatic firing mode switch ends up to be more confusing or not. It makes sense though that a shell that is primarily targetet vs infantry also targets infantry, or at least has the chance to do so.
We shouldn't forget though that there is no 'target infantry only' button in the games. The canister shot will target tanks if necessary. This is about saving a click for the player. We actually already have a somewhat comparable example: TDs entering the field with and prioritizing vehicles from the start, while all other tanks don't have to. It makes sense intuitively, and saves a click for the plaEr as well. Yet, there are no huge discussions about it apart from some people complaining that they need to get their muscle memory changed.
I'm not arguing here over either of the two things that I mentioned as examples for consistency, so there's no need to get defensive about it.
But again, you mentioning consistency in your reply, all of the TDs enter the field with vehicle prioritization turned on. Cool. It's consistent and intuitive.
Disabling vehicle prioritization when switching to AI shells on the M-42 and the M-42 only? It might be intuitive, but it's not consistent.
Someone switches their M-42 to AI rounds and forgets to turn target vehicles off the first time they use it. Big deal. They won't make that mistake again. In fact, if they play USF, they might even be familiar with how the game already treats shells and not even have this issue.
Or lets say you turn off vehicle prioritization when you enable AI shells on the M-42 and someone gets used to it (I'm assuming this is a change for new players because most people just learn to press the button that switches prioritization by muscle memory). How about that new player switches to playing USF and gets confused why the Sherman doesnt automatically turn off vehicle prioritization? Literally the same problem just in reverse lmao.
Or you change both of them but not the ISU because it can actually damage tanks too. And then inconsistency. OR you just change all of them and I guess the problem goes away at that point...
Point is; if you want to mess with this, do it to all of them if you value your intuitiveness and consistency.