To put it another way, are there better explanations for Noda's victory on offer other than (a) a deeply rooted death wish among DPJ Diet members, or (b) mental overload among said Diet members leading to malfunctioning of the intellectual processes?

Yes, there is. Maehara may have been the most popular choice according to opinion polls, but there was the donation problem and questions whether Maehara had really learned his lessons from his short and troublesome first stint as party president five years ago. Mabuchi and Kano had little chance from the very beginning and then it came to Noda versus Kaieda. The ways it looks the latter was by many regarded as Ozawa and Hatoyama's puppet. The allergy against Hatoyama and even moreso Ozawa evidently was stronger than any concerns about Noda's ability to serve as the party's face in an election campaign. In other words, Noda was the better of two less than ideal choices. This leaves us with the curious situation that now both LDP and DPJ are led by politicians that are very much alike, even though Tanigaki has been more moderate in his positions than Noda (at least until he became LDP president).
Both served as finance minister, are in favor of tax hikes and often criticized for lacking charisma.

Best,
Chris

Chris Winkler, PhD
Senior Research Fellow
German Institute for Japanese Studies (DIJ) Tokyo