Just some scattered thoughts today:
1. Rashi 9b explained that when the agent is zocheh in the shtar shichrur for the eved, he won't be free immediately, just that the zechiya prevents the owner from changing his mind. This was the opinion of the Rif. Tosafos 9b points to rashi 13a who changed his tune and says that if the shtar would be around and the shliach would be zocheh for the eved, he would be free immediately and there wouldn't be any issue of gett l'achar misa. See Rambam that the issue in not being zocheh immediately is not a lack of power of zechiya, rather "te'in k'zchi" means to be zocheh only in regard to preventing retraction, but if given full permission to be zocheh, all would agree that it is effective immediately. The Rosh (13) argues on Rif and says zechiya works as if it reached the slaves hand since it is working through shlichus and cannot be effective partially. He proves this from the gemara 12b that implies as soon as the shliach receives the shtar shichrur the slave could not eat teruma.
2. Rashi on the mishna 13a says that a gett cannot be effective after the husband dies since he lost his writes over her with his death. Rashi on 9b also says by a shtar shichrur that it can't go into effect after the master dies since the eved already left his reshus and now is in the reshus yorshim. However, Rashi in kesubos 2b says that the reason a gett cannot be effective after misah is because אין המתים מגרשים. There is a subtle yet significant difference. The idea of "ein hameisim migarshim" implies that since the dead husband can't divorce, and if the gett is in the hand of an agent, the agent can't do what he himself can't do. But, the Rambam holds that if one appoints an agent and then becomes a shoteh, the agent CAN divorce even though the husband himself can't. Ketzos (188:2) points out that here too we should allow the agent to divorce even though the husband is dead and can't do it himself? The ketzos explains that is why we need rashi to give another rationale, that the woman and eved has already left his jurisdiction so that he had no power to do anything over her. See Tosafos D.H. lo yitnu, that we have 2 mishnayos teaching ein get l'achar misa, to teach both these concepts: 1. in a case of shliach, he can't do what the husband cannot do (and a dead husband can't divorce). 2. even if she has the gett, it can't go into effect after he is dead because she leaves his domain through his death.
3. Tosafos says that although when Reuven sells a debt that he is owed to Shimon, he can still be mochel, but if he gives it through ma'amed shlashtan he cannot be mochel. Tosafos usually explains that the rationale that Reuven can be mochel a shtar that he sold is because kinyan shtaros is only d'rabonon. Even though ma'amed shlashtan is only d'rabonon (Tosafos 14a), Tosafos holds they empowered the receiver more than through a sale. But the Ran (in kesubos) explains that Reuven can be mochel because he only sold the shibud nechasim but retained the shibud haguf, if by ma'amed shlashtan Reuven can't be mochel, it must be because chazal actually transfer the shibud haguf (this is a chiddush that chazal would do such a thing!).