לפיכך כשסועד אדם בלילה הזה צריך לאכול ולשתות והוא מיסב דרך חירות
The Rambam and Rosh disagree whether someone who ate matzah or drank the 4 kosos without haseiba fulfills his mitzvah or not. The Brisker Rav (does everybody have a Brisker Haggdah these days?) explains the underlying issue as follows: is haseiba an independent mitzvah, or just a tnai in how to fulfill the mitzvah of matzah or kosos? If it is an independent mitzvah, then one can fulfill matzah or 4 kosos without fulfilling the mitzvah of haseiba. If it is a tnai, then missing haseiba undermines the mitzvah of matzah or 4 kosos itself and you have eat and drink all over again.
Rav Soloveitchik had a tremendous question on this hesber. The gemara has a din that if bandits force someone to eat matzah, he is yotzei the mitzvah. If haseiba is an integral part of the mitzvah, how can that be? It’s one thing to force matzah down someone’s throat – surely the gemara is not speaking of a case where they force the person to sit with haseiba as well?!
As we know from the haggdah, Rabban Gamliel teaches that if you fail to talk about pesach, matzah, and maror you are not yotzei the mitzvah. What mitzvah? The Rishonim (Rambam, Tos, Rambam) debate whether Rabban Gamliel meant the mitzvah of matzah, maror, and pesach, or whether he meant the mitzvah of haggadah. In other words, are pesach, matzah, and maror necessary to fulfill the mitzvah of sipur yetzias Mitzrayim, or is sipur yetzias Mitzrayim a necessary ingredient of the mitzvos of pesach, matzah and maror?
Based on this chakira, the Rav suggested (See Moadei haRav, also R' Genack's sefer Gan Shoshanim siman 20) that eating matzah without haseiba is good enough for a kiyum of matzah qua matzah, but the Rosh holds that one must still eat again in order to fulfill the mitzvah of matzah qua an essential element of the mitzvah of haggdah.
B) The gemara (108b) quotes a machlokes regarding whether ketanim are chayavim in 4 kosos:
ת"ר הכל חייבין בארבעה כוסות הללו אחד אנשים ואחד נשים ואחד תינוקות א"ר יהודה וכי מה תועלת יש לתינוקות ביין
ואחד התינוקות. שגם הם נגאלו
I saw suggested (see here) based on this Rashbam that the hesber of the machlokes is whether the sevara of af hein hayu b’oso ha’nes creates a chiyuv on ketanim or not. The problem is that the Mishna in Megillah (19b) quotes R’ Yehudah’s view that a katan can even be motzi a gadol in krias hamegillah. Many Rishonim (Ramban, Tos, see Tos R’ Akiva Eiger there) explain that a katan becomes mechuyav just like a gadol because of af hein. If so, how does R’ Yehudah’s view there jibe with his view here that af hein does not create a chiyuv in 4 kosos?
I have a simpler question. On that same amud in Pesachim the gemara writes that women are obligated in 4 kosos because of af hein. Rashbam explains:
שאף הן היו באותו הנס. כדאמרינן (סוטה דף יא:) בשכר נשים צדקניות שבאותו הדור נגאלו
According to Rashbam (as Tosfos on the spot notes and takes issue with) the sevara of af hein involves being a cause of redemption, not just a participant. This sevara cannot apply to ketanim, so the Rashbam must mean something else entirely when he says שגם הם נגאלו.