Ta'aroves chameitz and the shiur for bittul

Chameitz is not bateil in 1/60 like most issurim - even a "mashehu", the smallest amount of chameitz, if found on Pesach, is not bateil. The Rishonim give different reasons why this should be so. The Rosh explains that chameitz is treated more stringently because it is an issur kareis. However, the Rambam writes that the reason is because chameitz is a "davar she'yesh lo matirin" - something which will become permissable of its own accord at some later time is never bateil, because why rely on bittul when it will eventually be permissable anyway. The Aruch haShulchan writes that if the chameitz in question would spoil over Pesach, then the Rambam's reason would not apply. I had understood the Rambam as meaning to explain the takkanah of Chazal - since chameitz as a category of food usually will become permissable, therefore the Chachamim were gozeir a smaller shiur of bittul. According to my original reading, even if the particular food in question will spoil, the gezeirah still applies.
If the chameitz in question is not an issur kareis, then the Rambam's reason still applies, but the Rosh's does not. This would be the case for chameitz nokshe as well as for a ta'aroves chameitz according to the opinion of the RI"F (see post from Wednesday).
The Rambam interestingly adds (Ma'chalos Asuros 15:12) that even though davar shyesh lo matirin is usually permitted in a ta'aroves of min b'aino mino, by chameitz we are strict because the Torah commands "kol machmetzes". This is the same pasuk the Rambam cited with respect to a ta'aroves with less than k'zayis b'kdei achilas pras, where we ran into the difficulty that it follows the derasha of R' Eliezer and not the Chachamim.