sheni l'tumah - d'oraysa or derabbanan?

This post was instigated by my brother-in-law, who noted it makes a good yediya to post on my blog, so here it is. There is a din by ma’aser sheni that “mechitzos koltos”, once ma’aser enters the walls of Yerushalayim, it must be eaten there. Of course, if you remove kedushas ma’aser sheni, then the food could theoretically be eaten anywhere, but the halacha is that you cannot be podeh ma’aser sheni in Yerushalayim. The only exception is if the food becomes tamei, in which case, based on a dersaha, pediya is allowed even in the city. The Yerushalmi has what I thought is a fantastic chiddush in the name of Bar Pada (daf 18) – if food becomes tamei only as a “vald hatumah”, i.e. tamei from a rishon so it is only a sheni l’tumah, since it is tamei we allow pediya, but since min haTorah this food is still tahor, we still apply the law of mechitzos koltos and the redeemed food must be eaten in the city. Seems pretty clear from this din that a sheni l’tumah is only a din derabbanan --I am tempted to add exclamation points here, because I certainly was baffled when I learned this. My BIL was kind enough to point out my ignorance of the Rambam, who indeed paskens exactly like this gemara (hil ma’aser sheni perek 2) and writes that a "vlad hatumah" is only m'divreihem (derabbanan). As he remarked, doesn't picking up interesting yediyos like this make learning Yerushalmi worth it? (The truth is no, yediyos like this just leave me perplexed, and I don't have enough time to devote to delving into them). Not being quick enough on my feet when talking this over 5 minutes before seudah shlishis (we were both at another relative's bar mitzvah), I had to wait until this morning to e-mail my BIL back to explain upon further reflection why taking this Rambam k'pshuto is next to impossible! The Mishna in Sota (5:2) proves from pesukim that a sheni l’tumah causes a shlishi l’tumah – if food falls into a kli cheres oven which has a sheretz dangeling in it, the sheretz is matamei the oven (rishon), the ovens airspace is matamei the food (sheni), which in turn the Torah says is ‘yitma”, causes other things to become tamei. All this is on a d’oraysa level, as the Rambam writes in hil Avos haTumah ch 11. So how can you say a "vlad hatumah" is only derabbanan?! This difficulty is raised by the Mareh HaPanim right on the daf, who changes the girsa in the Rambam to fit the rule in avos hatumah. But, as my BIL pointed out originally, the meforshei haRambam take the words and girsa k’peshutan, so what in the world do they do with the Mishna in Sota?