1) The M.B. (643) quotes a machlokes haposkim whether one should say leishev ba’sukkah after the borei pri hageffen of kiddush on Shabbos or Yom Tov morning. Since just drinking a cup of wine does not ordinarily constitute a keviyus seudah that would require sukkah, there are those who argue that introducing a leishev ba’sukkah between borei pri hagefen and drinking constitutes a hefsek. Better to say the leishev ba’sukkah after the motzi, which does make for real keviyus.
2) B’pashtus, a piece of cake is just a snack and not something eaten with any sense of kviyus; therefore, it would not require a sukkah. If you do eat it in the sukkah, making a bracha of leishev b’sukkah would be problematic. The M.B. (639:16) suggests that you can avoid the problem with a tziruf: eat the cake and linger in the sukkah afterwards for some time. Since there are poskim who hold you can say a leishev ba’sukkah just for remaining in the sukkah (even without eating), and there are poskim who have a minimalist view of what eating constitutes keviyus, you can put 2 and 2 together and have enough to rely on to make a bracha.
R’ Shlomo Zalman comes up with the following: just like the leishev ba’sukkah, if not necessary, may be a hefsek between kiddush and drinking, so too, the leishev ba'sukkah, if not necessary, may be a hefsek between the bracha of mezonos and the cake. True, you will linger in the sukkah afterwards, but at the moment you are eating the cake that is not going to help. Therefore, suggests R’ Shlomo Zalman, the best course of action would be to say the mezonos and eat some cake and only afterwards, once you have taken a bite and avoided any hefsek, say the leishev ba'sukkah.
By the same token, when a women sits down to eat in the sukkah, since she has no chiyuv, she has no real reason to recite a leishev ba’sukkah. True, minhag Ashkenaz is to do so, but it comes at the expense of creating a hefsek between the birchas ha'ne’henin (the mezonos or hamotzi or whatever) and her eating. Therefore, just like in the above case, it may be best for a woman to take a bite of her food first after the birchas ha'ne’henin and only then recite the leishev ba’sukkah.