Joseph:
You cannot create a mitzvah as you are about tying the left shoe first. There are many practices that are in the category of minhag, not mitzvah. It is problematic, perhaps assur, to give them mitzvah status.
Next – your quotes from Rambam and Shulchan Aruch Harav must be understood in context. If the potch is an expression of revenge at a child for having disobeyed, it is ossur. No one ever, ever gave any leniency for that. If it emanates from anger, it is ossur. The Brisker Rov ZT”L stated clearly that the first potch, under specific conditions might be able to be considered muttar. But the second one was never permitted, and was ossur min haTorah. Any discussion of discipline must be able to teach the talmid, not punish. In fact, the concept of עונש does not exist in הלכות חינוך. That includes the potch. If the child learns to associate the misbehavior with negative consequences, that can be educational. If the child learns to consider the one administering the punishment as bad or hateful, that is not chinuch, and the potch was not muttar at all.