Though I don't really care about arguing over "who rules in the studio" because we really cannot form TRUE opinions, not having been involved...we're looking in from the outside, so I personally think it's a waste of time to even think about this in such depth when you cannot possibly be coming from the same place as the band. Just my personal feelings.
Anyway...despite this opinion, I do agree that they should have gone ahead with a double album. It probably would have sold just as many copies (ie, the same group of people who bought the single album would have been willing to buy a double). I almost tend to think an 18 track single album might be more overwhelming to people in general. At least if, say, 24 tracks are divided onto 2 discs, they feel like they'll get a rest in between
. Especially, as mentioned, if they were going to put out the set with the bonus CD anyway...