Interesting thought experiment.
I didn't find the previous cases compelling but I I don't know whether you've got it wrong here. However, I'll play demon's advocate and make two quick points:
First, why do you assert that, because the push was hard enough to make Player B fall, Player A has necessarily used 'excessive force'? Tinney's remarks in the Trengove case suggest that you can tackle someone - presumably, to the ground - 'legally' without using excessive force.
If that's the case, it's not clear to me that Player A would be deemed to have used 'excessive force' for simply pushing Player B hard enough to make them fall over.
Second, why do you assert that Player A, in the act of pushing Player B over, has breached his 'duty of care'? I have only read the relevant Appendix to the rules, so I'm simply assuming that duty of care relates to taking reasonable care to avoid forseeable actions. (If the AFL has defined it in the rules, let me know!). It seems to me that one could coherently argue that while Trengove may have breached his duty of care when tackling Dangerfield, Player A has not necessarily breached his duty of care to Player B, as one might think there are important distinctions to be made between the two cases.
For example, Player A does not in any way impede Player B's ability to regain balance - Player A does not drive Player B into the dirt, but simply pushes Player B - or impede Player B's ability to break their fall (by, say, holding onto their arm at any stage).
I'm not laughing. I enjoyed thinking about your analysis, though.