is zman gerama a ptur by mitzvos derabbanan?

Whether or not women are exempt from a mitzvah derabbanan which is zman gerama seems to be a machlokes Rishonim.  The gemara (Brachos 20b) according to our girsa suggests that one might have had a hava amina that women are exempt from tefilah because it is zman gerama, kah mashma lan the Mishna that this is not true.  Rashi writes that this girsa must be wrong.  Since tefilah is derabbanan, women are obviously obligated, whether it is zman gerama or not.  Tosfos disagrees and proves from various other cases (e.g. hallel) that the ptur of zman gerama applies to dinim derabbanan as well. 

What is the crux of the machlokes?  The reason why we must observe dinim derabbanan (b’pashtus) is because the Torah commands, “lo tasur.”  Yesh lachkor whether this means we treat each particular din derabbanan as if it was commanded by the Torah, or whether “lo tasur” is a general umbrella obligation not to disrespect Beis Din by ignoring their laws.  If the first approach is correct, then Tos would seem to be right – the same rules, such as zman gerama, that apply to dinim d’orasya should apply to dinim derabbanan.  According to the second approach, that all dinim derabbanan fall under one general umbrella of “lo tasur,” since that obligation is a lav, not an aseh, there is no exemption of zman gerama.

Chanukah is fast approaching!  The gemara tells us that women are obligated in ner Chanukah because “af hein hayu b’oso ha’nes,” they shared in the miracle of being saved from the Greeks (other Rishhonim learn that they were the cause of that miracle.)  According to Tosfos, that women are exempt from mitzvos derabbanan that are zman gerama, I understand why the gemara needs this reason of “af hein…” to tell us that they are obligated in ner Chanukah.  But according to Rashi, that there is no ptur of zman gerama by dinim derabbanan, why does the gemara need this extra reason to obligate women???