I thought this whole board's warcry was "consistency!". Target tables are a no go because interaction between squads becomes inconsistent, faction asymmetry is ok but only when everyone has consistent access to the same tools, and so on.
Tell me how consistent it would be for a shell switch changing the target priority on your selected unit? Sure, the M42 won't be doing any damage against tanks with its AI rounds, but neither does the Sherman, and it keeps its prioritization settings anyways.
And if you do make those consistent between each other, then the ISU is the odd man out because its AI shells CAN do damage to tanks (at least as far as I remember, unless I'm mixing it up with the KV-2).
Hell, though, if I am wrong about the ISU then screw it, disable vehicle prioritization while in AI shells mode for all of them. I don't really see the downside.
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.