@chaley: This seems to do it - I changed #sony_sort to:
{author_sort} - {series}{series_index:0>2s| | - }{title}
and it's displaying correctly in the "Added Order" sort option in collections. Now if only "Added Order" had the little alphabet along the side like Title & Author sort do to make browsing easier, I'd be thrilled. :P In the meantime, this will do! Thank you!!!
Oh, and I'm glad my "unrelated problem" earlier regarding the prequel being out of order was useful.
And as long as I have people's attention - when I view my "all books" option, it default sorts by author, and, I think courtesy of jackie_w's plugboard I'd implemented earlier, it does group series together - but it orders them in reverse order. I don't use this view hardly ever, but I'm still curious as why it's doing this (or, more likely, what I did by accident to make it do this), if anybody has any thoughts.