The MG”A (siman 491) has a safeik if one is permitted to eat chameitz before ma’ariv and havdalah in the case where one starts a meal before night on the last day of pesach and it continues past tzais. His psak is l’kula based on the fact that the issur chameitz is independent of kedushas Y”T, as we see from chol hamoed – no kedushas Y”T (as we see from the fact that no havdalah is recited), yet there is still an issur matzah. During tosefes which we add after nightfall there would be kedushas Y”T, but only with respect to the issur of doing work, not viz. chameitz. I spoke about this issue last night in shule. The MG”A raises a fundamental question – whether tosefes is a generic issur melacha, or an extension of the entire kedushas Y”T. Tosfos (Kesubos 47) has 2 deyos whether the issur of “ain m’arvin simcha b’simcha” (one cannot get married on Y”T because the simchas chasan interefes with simchas Y”T) applies during tosefes Y”T – again, the issue would seem to be does tosefes have a full kedushas Y”T including simchas Y”T, or just an issur melacha. The Divrei Yechezkel (#45) quotes a kasha from R’ Chaim: we learn out through a gezeirah shavah that just as on leil pesach one is obligated to eat a k’zayis matzah only after nightfall, so too one must eat a k’zayis in sukkah only after nightfall. By Pesach, one cannot fulfill that obligation during tosefes, because the chiyuv does not depend on the kedushas Y”T of pesach, but specifically on it being night – e.g. the kedushas Y”T of the chag applies during the day, but there are only chiyuvim of pesach, matzah, and maror at night. However, asks, R” Chaim, the same is not true for sukkah – there the chiyuv of sukkah depends on kedushas Y”T, as we see from the fact that there is an obligation of sukkah anytime you have a meal during the chag. So why not eat during tosefes, and just after nightfall make sure to consume a k’zayis to fulfill the chiyuv of eating k’zayis during real night? The entire kasha is predicated on the assumption that tosefes has a full kedushas Y”T which would obligate one in sukkah; if one assumes that tosefes is just an issur melacha, then one cannot use the sukkah during that time period. There is one monkey-wrench in this whole analysis. Tosfos (Pesach 99b) writes that we learn from “b’layla hazeh” that the mitzvah of korban pesach can only be done at actual nighfall, but not during tosefes. If tosefes is just an issur melacha and not a full kedushas Y”T, of course you cannot be mekayeim the mitzvoth hachag during that time period – why do you need a separate gezeiras hakasuv to teach this halacha? (R’ Elchanan asks this in Koveitz Shiurim; in the new Ch HaGRI”Z stencil in the sugyos section there is a teitutz from the GRI”D, but it is a different approach.)
One side note: R’ Chaim held that chol hamoed has a kedushas Y”T with a heter melacha – seems to be against this MG”A.