1. how did the torah force a person to be oiver on lo sosiru by not allowing him to break a bone that contains a kezais of marrow?
2. who is liable if meat is left over?
3. if a person's korban became tomei so that the he was yotze the scechita but was unable to eat, would he be able to join another group?
1. Not only does the Torah make something forbidden which will lead the annulment of another Mitzvah, but even the Rabanan sometimes do so (as we find on 84b, "Mishim Paka"). The person who follows the Rabbinic institution is considered an "Ones," and is not considered to have transgressed a Lav. In the case of a Biblical institution, he is not even an "Ones," because the Torah itself effectively exempts him from eating the marrow and permits him to leave it over.
2. All of the members of the Chaburah of this Korban Pesach are Chayav if any of it are left over (Michas Chinuch 8:1, based on Pesachim 63b, see Rashi there DH Kol Bnei Chaburah). Even though one person left over his own portion, they are all responsible, since the members of the Chaburah eat the Korban together and are responsible to ensure that no meat is left over. The Kesef Mishnah (Hilchos Korban Pesach 9:6) says that if some finish and leave early, they will not be Chayav for Lo Sosiru (since they were allowed to assume that those who remained would finish it themselves -- i.e. they are considered "Ones.") Presumably, if a Pesach was eaten in two Chaburos, in two different places, the same would apply. Only those who are eating the PEsach together are responsible for each others leftovers.
3. A person cannot be enlisted to two Pesachim (88b); if Zerikas ha'Pesach was already performed for him, he cannot change Pesachim. (According to Rebbi Shimon on 89a, one can leave a Minuy to a Pesach even after Shechitah, as long as he does it before Zerikas ha'Dam. However, before Zerikas ha'Dam he has not yet been "Yotzei" bringing the Pesach.)
Best wishes, Mordecai
Thank You for your reply regarding the Issur of Baal Tosiru devolving on all the members of the chaburah. I was just wondering how the Mitzvas Aseh of serafah would be fulfilled? Since there is now an individual chiyuv on each member to burn the Nosser, especially if one holds Kimu V'lo Kimo, one could not make a shaliach, and everyone would have to burn the Nosser simultaniously.
I don't see any reason that it should not be possible to appoint a Shali'ach for this. It is not in the category of "Mitzvos sh'bi'Gufo" like Tefilin and the like, which cannot be done by a Shali'ach. For instance, a man who divorces an Anusah (a woman he raped), thus transgressing an Isur Lo Sa'aseh, should be able to be Mekadesh her again via a Shali'ach in order to be Mekayem the Aseh (see Makos 16a).
(It also seems that even if a Shali'ach was not appointed to burn the Nosar, as long as the Nosar was burned in any manner, the Aseh is fulfilled and one no longer will receive Malkus for transgressing the Lav. The Chiyuv to burn the Nosar is not placed exclusively on the one who transgressed the Lav (on the "Gavra"); rather, as long as the Nosar is burned (the "Cheftza"), it should exempt him from Malkus.)
Best wishes, Mordecai