There is no scholarly consensus about this.
There are good reasons for thinking that the Ratnavali was written quite a bit later than the MMK and the Vig; that it was written either by a different author by the same name, or incorrectly attributed to the 'original' Nagarjuna.
It is, as you say, clearly a Mahayana text. It seems probable that the 'Nagarjuna' who wrote the MMK and the Vig was a Mahasangika.
In any case, we do not know.
But one thing is for sure: there is a narrative about all of this which although clearly speculative, purports to have more coherence than it should.
The effort of some academics to re-brand the Nāgārjuna who authored the MMK as a non-mahāyānika is impossible to substantiate and also quite pointless from a practical point of view. Regardless of who specifically authored which texts, Nāgārjuna has been seen as the grandfather of all things Mahāyāna for over 1500 years, and this perception isn't going to change just to satisfy the historical revisionism of a few mādhyamaka inspired Pāli Buddhists. Plus, any Pāli Buddhists who find that mādhyamaka resonates with them can find all the mādhyamaka they need in the Pāli Nikāyas, the Theravāda Paṭisambhidāmagga, and the writings of Ven. Ñāṇananda.
All the best,
My point is a/that Nagarjuna is appropriated in particular ways by all sides; not just Pali oriented academics. And b/ we don't really know who 'Nagarjuna' was, which school he was from and which texts 'he' did or didn't author.
Therefore, no one of any persuasion has any basis for appropriating his work to fit into a particular framework.
I mean, you're quite right that it's impossible to substantiate. So, the question must be left open.
Later Madhyamikans are a different story.
I don't think this changes N's grandfather status in the Mahayana traditions. I don't really see it as an issue if one deals with each text on its merits.
Incidentally, the argument that the Ratnavali is authored by a different person comes from Tillmann I think. I forget his reasons, but it is probably a philological claim. It is odd that bodhicitta is so prevalent in the Ratnavali, but so conspicuously absent from the MMK and Vig.