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!
Exploring Theravāda's connections to other paths. What can we learn from other traditions, religions and philosophies?