Dredging the harbour to build a better port, eh,
Eomer?
I think you have to consider the placing of Shelob in terms of the movie concept PJ had for LotR: action/adventure/blockbuster flic. As a creative artist, he has the right to develop his own interpretation, as long as it is marketed as his version of events, of course.
The yardstick for placing Shelob then becomes where will her inclusion best appeal to the kind of audience which favours action/adventure/blockbuster flics.
I think it would just be *too* scary for the general members of this audience (where the XY chromosomes hold the majority--at least, it seems so to this minority XX member) to contemplate an ending with a female, loathesome as she is, to get the upper hand on our young hero. It is inconceivable that a female, even an evilly vile one, could come off with the big climax. Orcs carrying off Pippin and Merry are no threat to the masculinity of those youthful hobbits, but a female, well, PJ had enough to deal with representing Frodo's kind of heroism without inviting that kind of disasterous denouement.
