This is a really excellent thread.
Ven. Huifeng has got me wondering if there are good philological reasons to support the claim that the Ratnavali has a different author to the MMK and the Vigrahavyavartarni. And perhaps the possibility that it is a later text.
It seems to be taken as a given in most scholarship that even though there is quite a lot of ambiguity about other texts, these three are taken to be authentically attributed to this mysterious figure "Nagarjuna".
But I must say that prima facie, without hearing his reasons, I can definitely understand Gombrich's sense that the MMK may not be a Mahayana text. In terms of content, there is nothing distinctly Mahayana asserted. I think the Vig is very similar in this regard, and it seems very plausible that both were written by the same author...if only because there is such a cogent philosophical continuity between them.
Yet the Ratnavali is manifestly Mahayanist in orientation, with constant references to the Mahayana as a vehicle, as well as constant accounts of bodhicitta.
It seems like something of a lazy inference to say: well, the first two are about metaphysics and epistemology, and the third is about ethics; that is what explains the difference.
If it is the case that the Ratnavali has a different author, then what does this do to Walser's thesis? i.e. Which Nagarjuna are we talking about and which context!