You’re asking a kashya, then, based on assumptions which are quite questionable. According to your assumptions, the chilluk between cases where R’ Tarfon holds of dayo or doesn’t should have nothing to do with heicha d’mifrach kal vachomer.
It actually does make sense that something which is more chamur should also be more chamur elsewhere, and it also makes sense to never say a kal vachomer because of a possible unknown pircha (a reason given for ein onshin min hadin) but the bottom line is that the Torah gives guidelines as to when to say and how far to say a k”v. R’ Tarfon and the Rabbanan disagree about the guidelines.
Where I though you had a valid question was in categorizing the two halachos vs. os achas as dayo, as opposed to calling the two halachos a tzad chamur and therefore a pircha (I think this can be answered, but it’s a good heoroh).