Probably someone has a better answer than I do, but FWIW, I find S.
flava to be distinct from S. oreo. in terms of the strongly reflexed
column paired with a flat, horizontal lid. Flower smell in S. flava is
always very pronounced, and S. oreo seems to me to typically have a more
dusky, mottled appearance with frequent red spotting.
I agree that S. rubra vs. S. alata can be a problem. The typical
"pencil" tubed S. rubra is easy to distinguish, but there are S. rubra
subspecies, varieties, populations or what have you that look much like
S. alata. I have not found this to be so much the case in the wild as
in greenhouse or home grown plants. It also seems to me that S. rubra
pitchers tend to change during the growing season with early pitchers
often etiolated and more similar in shape to some of the S. alata. I
look at the shape of the lid (longer=rubra), to some lesser extent the
degree to which the column is less reflexed and continuous with the lid
(rubra), and the position of the lid more over the mouth of the trap
(rubra). S. alata also seems to me to be a more robust, sturdy pitcher.
S. rubra typically produces red flowers while S. alata typically
produces yellow flowers (I wouldn't dare say always).
FWIW
Tom in Fl
PS. I seem to remember that one of the keys talks about the pattern of red
veination in the pitcher, but damned if I can see consistent distinctions.