Dear A. J.
I agree with most of your characterisation of the current travelling wave (TW) theory. However, one aspect that is disputed by several theoreticians is the assertion that it is the impedance gradient that determines the direction of propagation of the TW. In the TW theory as given by, say, de Boer (1996), the TW can propagate in either direction along the BM. If a source is located at a point along the BM, it can (in this theory) propagate as a TW in both directions along the BM, provided it couples to TW motion. This is the explanation given for both distortion product and reflection otoacoustic emissions. Similarly a hypothetical BM with no impedance gradient at all would still allow a TW propagate from base to apex along the BM. So on this theory the TW is a true wave, the BM velocity obeying a version of the wave equation.
Contrary to the above, some authors state that it is the BM impedance gradient that determines the direction of TW propagation (e.g., Pickles, Intro to Physiology of Hearing). This belief may have arisen from an experiment by Wever & Lawrence (1954) which seemed to show that a source located at the apex caused a TW to be launched from the base to apex. However, Lighthill (1981, p. 178) argues that this was because the Wever & Lawrence apical source coupled to the fast (compressive) wave, which then progated to the base where it encountered the asymetrical impedances of the oval and round windows, which thus launched the TW, which is excited by the asymmetric "push-pull" of the two windows. So it was not the BM gradient that caused this effect, but the nature of the source (it coupled to the fast wave), and the impedance asymmetry.
Thus, according to Lighthill, de Boer, and others (eg. Shera, I think), the BM impedance gradient does not determine the TW direction: the TW can propagate away from a source like any other wave. (There is a complication that the TW cannot propagate along the BM in the mass-controlled region, but that's a separate issue.)
de Boer, E. (1996) ‘Mechanics of the Cochlea: Modeling Efforts’ in P. Dallos, A. N. Popper, R. R. Fay (Ed.), The Cochlea, Springer-Verlag, New York, pp. 258-317.
Lighthill, M. J. (1981) ‘Energy flow in the cochlea’ Journal of Fluid Mechanics, 106, pp. 149-213.
Shera CA, Tubis A, Talmadge CL, de Boer E, Fahe PF, Guinan JJ Jr.(2007) Allen-Fahey and related experiments support the predominance of cochlear
slow-wave otoacoustic emissions.' J Acoust Soc Am. 2007 Mar;121(3):1564-75.
Shera CA, Tubis A, Talmadge CL.'Do forward- and backward-traveling waves occur within the cochlea? Countering the critique of Nobili et al.' J Assoc Res Otolaryngol. 2004 Dec;5(4):349-59.