It is easy enough to make the season group optional:
(^.*?)(?:\Ws(?:eason )?(\d{1,2}|[ivxlcdm]{1,5}))?\Wp(?:art )?(\d{1,2}|[ivxlcdm]{1,5})\W(.*$)
using a non-capturing group ((?:...)) plus the 0 or 1 quantifier (?). I did have to make the first group non-greedy to prevent it from matching the season section of the name.
I also made the eason and art optional strings into non-capturing optional groups instead of character classes.
Result:
>>> import re
>>> p=re.compile(r'(^.*?)(?:\Ws(?:eason )?(\d{1,2}|[ivxlcdm]{1,5}))?\Wp(?:art )?(\d{1,2}|[ivxlcdm]{1,5})\W(.*$)', re.I)
>>> p.search('miniseries.season 1.part 5.720p.avi').groups()
('miniseries', '1', '5', '720p.avi')
>>> p.search('miniseries.part 5.720p.avi').groups()
('miniseries', None, '5', '720p.avi')
>>> p.search('miniseries.part VII.720p.avi').groups()
('miniseries', None, 'VII', '720p.avi')