Cyrus,
I have some difficulty in following the reasoning.
At what chronological height Jordanes placed the migration of the Goths to the continent will we never know. He would have to ask himself if he was still alive.
Let's go back to the starting question: do you ask if there is any linguistic evidence that testifies incontrovertibly to the origin of the Goths from Scandinavia, right?
No, strictly speaking, no: we have no "smoking gun" that can be taken as a sure and objective proof of all this. We have only a few indirect indications that could make a Scandinavian origin or a closeness to the Scandinavian context plausible: some phonetic phenomena common to eastern Germanic and Norse / Northern Germanic, and the lexicon of some Scandinavian dialects, now extinct.
Jordanes information remains, so to speak, isolated, and you are right when you say that he himself passes on somewhat confused historical-geographical notions, being an epitomator working on lost previous works by non-Goth authors (including Cassiodorus), who drew on their turn to other traditions and with other purposes.
It would however remain to understand why he feels compelled to report this topos of Scandinavian origin in his writing. The same thing would have happened later in the Lombard context with the anonymous Origo Gentis Langobardorum and with Paolus Diaconus with his Historia Langobardorum (which perhaps in their turn took on a Jordanes model?). Although the news may not be true or not entirely trustworthy, it's clear that it was important for them to pass on this memory, but before abandoning what is now only a hypothesis - despite the fact that today's leading scholars of the Goth may think otherwise, like Wolfram and Pohl- something else is needed that defeats it openly: lack of evidence is not evidence of absence. Just as we admit that Scandinavia may have been reached by continental Indo-European groups two millennia or so BC. it doesn't even seem so out of the ordinary that around the beginning of our era there was a partial counter-migration towards the continent.
We need to agree on what we want to save from what Jordanes wrote. If we do not want to give credit to literary testimony, we must make do with what else is available, based only on material elements and archaeological evidence that makes us suppose that the first possible Goths are formed on the continent, with a first Germanic / Baltic nucleus, within the cultures of Oksywie and Wielbark, in Pomerania (however not far from Scandinavia) and from there they radiated southeast into the continent, ascending the Vistula. It is in the steppes of central and eastern Europe, north of the Black Sea, and partly in the eastern Balkans that the Goths are so called that they will flow back into Iberia and Italy, with the cultures of Černjachov and Sîntana de Mureș. That is to say when the Goths assimilate sarmatic, protoslavic and eastern Balkan elements including, in all probability, ancient Dacian and Thracian substrates. Notwithstanding the fact that the Goths = Geats equation has always seemed to me an operation of historical erudition advocated by elites, rulers and writers (not therefore a collective phenomenon present in the common imaginary), if I were to hypothesize when the Goths began to imagine themselves as Geats, I would pause more than anything else on this phase of the late antique, not before.
After which I absolutely agree that Germanic, ethnic and / or linguistic elements were present and infiltrated in Central Europe even before the Goths, according to trajectories that are not yet well clarified. The first attestation of Germanic language (or better of a Germanic name) is that one in the Etruscan alphabet, affixed around the II sec. a.C. on the helmet of Negau, an older artifact dating back to around the fifth century. BC, found in present-day Slovenia.
https://balkancelts.wordpress.com/tag/negau-helmets/
Almost all Germanic loanwords are from the south, Germanic script (Runes) is from the south, Germanic sources themselves say that they emigrated from the south (Scythia Magna), ... I see nothing except modern nationalism about Germanic origin in the Germanic lands, Iranian nationalists also love to say that Iran is the original land of Iranian culture, Indian nationalists say the similar thing about India, ... they just don't want to believe that their ancestors migrated from another land and it doesn't matter for them what we read about them in Avesta and Rigveda.