Do you have a good explanation for the gemora's two okimtas: first the grapes and then the chartzanim,where it seems the gemara gives the same answer twice. So what did the gemora think (the hav amina) in the case of chartzanim. Why did it think that the answer of not having anything to eat but this chartzan (or these chartzanim) would be applicable, and if there is a difference in the two cases (obviously that chartzanim are not nutritious - although I am told that natural medicine actually uses grape seeds) then what is the gemara's rejection. I understand that in part this is a typical hav amina - maskana situation which we find in the gemara. But I do believe that there is some logical hidden sequence. This also goes to the question of what "hacha bemai askinan really means. It certainly doesn't always mean that this is the actual intent of the gemara.
dmartin, raanana,eretz yisroel
Tosfos (DH "Gabei" and "v'Gabei") is clearly bothered by your questions. He explains that Rava's (or Abaye's) answer is that Chartzan is coming to supplement Rav Sheshes' original answer, which was only difficult because it was apparently talking about grapes which would make the servant week if he could not have them. Rava (or Abaye) therefore replaced grapes with a single Chartzan, which would not seem to serve any nutritous purpose, thwarting this question. The Gemara thought, Tosfos explains, that a Chartzan was insignificant food. However, the Gemara is able to ask that a Chartzan could also be helpful nutrition. Alternatively, Tosfos explains, the Gemara is asking that the Mishna should be general, not just talking about a Chartzan. This question is a general question, that the Gemara should not paint itself into a corner unnecessarily and talk about a narrow case of Chartzan instead of any normal food that would strengthen a person. According to this understanding of the Gemara's question, the logical sequence is quite apparent. The Gemara tried to explain that the Mishna was talking about a specific case, but bows to a question that the intent must be broader. [A discussion of your points regarding the general ways of Hava Aminas, Maskanas, and Hacha b'Mai Askinans is beyond the scope of this forum.]
All the best,