Ta'aroves chameitz

OK, here goes the plunge into the world of chamietz and matzah...

Those who have been learning the daf will recall the machlokes (Pes 43) R' Eliezer and the Chachamim regarding ta'aroves chameitz. If a mixture contains more chameitz than the quantity of k'zayis b'kdei achilas pas (i.e. if you munch on that ta'aroves for the time it takes to eat a loaf of bread of 3 or 4 beitzim, you will have eaten at least a kzayis chameitz as part of the mixture) then everyone agrees you are chayav kareis - there is no difference between eating a slice of bread and eating a slice of bread in a mixture. The Tanaim dispute whether there is an issur kareis where the mixture contains **less** than this quantity of chameitz. R' Eliezer learns from the pasuk "kol machmetzes lo tocheilu" that you are chayav kareis, and the Chachamim argue.
The RI"F has a chiddush that even though the Chachamim hold there is no kareis, they do hold there is an issur of eating such a mixture.
The striking difficulty is the Rambam's opinion. The Rambam paskens (Chameitz 1:6) that if you consume a k'zayis chameitz within the shiur of achilas pras you have violated the lav of "kol machmetzes" but are not chayav kareis. (1) If there is a kzayis b'kdei achilas pras, eveyone - both R' Eliezer and Chachamim - say you are chayav kareis, because this is just like eating a slice of bread; why does the Rambam here say you are chayav only a lav? (2) The Rambam quotes the pasuk of "kol machmetzes" which is used by R' Eliezer for his drasha but rejected by the Chachamim. Since the Rambam paskens like the Chachamim (otherwise, even if the mixture had less than a kzayis you should be chayav), why does the Rambam quote this pasuk? No great answers yet for this one...