I have to disagree with Namdrol. The Buddha was an omniscient being. He talked about all kinds of magical things, like beings from the six realms, like going to see Baka-Brahma, etc., He had this power of co-location. That is part of the Pali.
On this we agree.
The Prajnaparamita, some of it, was discovered by Nagarjuna when it was given to him by a Naga.
Myth, legend, but not historical fact.
The Buddha had hidden, just like a terma, the prajnaparamita sutras. If you believe in buddhahood, which all buddhists would, then why wouldn't this magical possibility be real? Why couldn't he have taught the tantras in his co-located form, and those were kept hidden until much later?
He could have, but it is unlikely. In any case, the definitive Buddha is Samantabhadra, not Shakyamuni.
So that leads me to conclude that the written record is not reliable.
The why assume that part of the written record is reliable (i.e. traditions around the Nāgārjuna, nāgās, etc.)?
The oral account has all the contexts to explain why the methods, and culture of dharma evolved the way it did.
The so called oral accounts you have received are all based on Texts translated by Tibetans. I have researched this area extensively.
There is a saying in dharma, it is wrong view to hold that only hard facts proven by external sources are true, because you are locking yourself into a subject object dualism and the path is the opposite.
IMO, the tantras do not depend on any version of the historical record for their validity. My argument is that they are valid because their source is awakened, but I do not need that source to be Shakyamuni -- Virupa, Garab Dorje, Padmsambhava, Sachen, Jigme Lingpa, Namkhai Norbu, Khenpo Jigphun, etc, are good enough for me.