kaltuhu mechitzos and kedushas yerushalayim

As previously discussed, the din of kaltuhu mechitzos teaches that once ma’aser sheni has entered Yerushalayim, it can no longer be nifdeh if removed from the city but must be eaten in Yerushalayim. This halacha is the basis of the Ra’avad’s critique of a fairly well known Rambam. The Rambam paskens (Hil Bais haBechira ch 6) that the gemara’s debate whether “kidsha l’shayta” or “kidsha l’asid lavo”, whether Eretz Yisrael was endowed with permanent sanctity or whether that sanctity was null and void after the Land was destroyed, applies only to those areas outside Yerushalayim. However, writes the Rambam, Yerushalayim has permanent irrevocable sanctity. The reason for this distinction is based on the nature of the mekadesh – the Rambam poetically explains that Yerushalayim is sanctified by the presence of the Shechina, and just as the Shechina is permanent and unchanging, so too, the city remains sanctified for eternity. The Ra’avad disagrees, and while his final argument, “sod Hashem l’yereivav” leaves little room for debate, his other proofs are interesting. Ra’avad points to a gemara (B.M. 53b, cited by the Rambam, Hil Ma’aser Sheni 6:16) regarding ma’aser sheni which entered Yerushalayim and was then removed from the city, and then the walls of Yerushalayim were destroyed. The ma’aser sheni cannot be nifdeh because of the din of kaltuhu mechitzos, but at the same time cannot be eaten because there are no mechitzos left around Yerushalayim. Doesn’t the fact that ma’aser sheni cannot be eaten in Yerushalayim after the destruction of the city walls prove that the kedusha of the city has been nullified? The Rambam may very well have taken issue with the very assumption behind this question. The din of kaltuhu mechitzos may not at all be dependent upon the kedushas hamakom of Yerushalayim, but upon the existence of physical walls – literally, kaltuhu mechitzos, it is the walls themselves which hold the ma’aser back from pidyon. If ma’aser sheni had entered the city after the walls fell, then according to the Rambam even though there is still a kedushas makom to the place, the absence of walls cancels the din of kaltuhu mechitzos (see Moadim u’Zmanin vol 5, #348).
Just as an aside, it is worth seeing the Ra’avad’s language in Hil Bais haBechira because it reflects a certain ambiguity regarding entering the Har HaBayis. Ra’avad writes that there is no kedushas hamakom and therefore one who enters the area is not liable for an issur kareis – does that mean it is permitted, or just one incurs no issur kareis for doing so?