The question is not just hard to answer, it is impossible, since it's based on two related, undefinable standards: "the pronunciation" and "reasonable scholarly assurance". Let's take "the pronunciation" of English winter. It is pronounced [wɪ̃ɾ̃ɹ̩], at least in my dialect. However, I do know that it is also pronounced as [wɪntɹ̩], [wɪntɜ], and [wɪnɜ] depending on dialect and individual. There isn't a unique pronunciation, in fact, I vacillate between [wɪ̃ɾ̃ɹ̩] and [wɪntɹ̩]. "Pronunciation" is a vague abstraction which covers classes of acoustic events, and even one individual uttering a word a dozen times will produce a dozen different outputs. The differences may be grammatically insignificant and very hard to perceive, but they exist. The question presupposes that a word has a unique pronunciation for all speakers, which is not the case. So the question need to be sharpened to specify what you mean by "pronunciation", in particular, the level of detail that you require. In typical linguistic usage, "pronunciation" admits of a lot of imprecision, yet you see people here rejecting candidate words because of the scent of imprecision.
More severely problematic is the standard "reasonable scholarly assurance". What does that even mean? Is there "reasonable scholarly assurance" that winter is pronounced ˈwɪnˌtʰɹ̩ or that the Classical Arabic word is [ʔaʃʃita:ʔ]? What is "reasonable scholarly assurance" opposed to? -- no scholarly reason at all? What about "very likely, but not absolutely guaranteed". If you don't accept my representation of the pronunciation of the recorded sample of "winter", how do you show that mine is incorrect and that something else is correct? Suppose you uncover a wax recording of English "winter" made 150 years ago, what would be "scholarly assurance" that the resulting representation is "the pronunciation"? There is a difference between "absolute infallible certainty" and "reasonable scholarly assurance", yet you see people being confused between the two.
I contend that proto-Indo-European and proto-Germanic reconstructions are "reasonable scholarly assurances", though in instances somewhat less certain than assumed Classical Arabic pronunciations. The (Vedic) Sanskrit word ɦimjáh (himyáḥ) "winter" is probably the oldest attested word for a season passing a high standard of certainty. There is not a shred of evidence to reject the well-established word and phonetic interpretation, so it passes the sniff test for "reasonable scholarly assurance" (though you may need to know more about the tradition of preserving Sanskrit to be assured, in case you are not familiar with Sanskrit).
