hello99 –
Sorry but I believe you are wrong about the Ran. Look it over again. The Gemara in Menachos 22b which asks rhetorically, ????? ?? ????? ??? ????? ??????, ???? – is according to you, the maskana, which is absurd.
once Chazal forbade Bitul Issur l’Chatchila, we cannot dismiss a d’Rabannan just because we think the motive is not relevant
Sure we can, as long as we aren’t being mevatel the gezeira. We always do this.