While I can't deny the similarities between Hermes and Pan, the etymology of their name doesn't line up. Thus making this suggested origin hard to accept, given the lack of evidence.

This is a misunderstanding what the claim is. There is no claim that the words Pan and Hermes share an etymological root. The claim is that Hermes was some other god, derived from some other source, who was then given a backstory of being Pan's father, after which he took on some of the son's attributes. The ultimate source of the word "Hermes" and his origin as a deity is unknown - some scholarship say the Greeks invented him, some say no, but all sources seem to agree that his root is not in PIE. There were many other non-PIE cultures around at that time with gods of their own, and these would get borrowed cross-culturally.

Yes and no... some cultural reflexes remained very static indeed, e.g. Sky Father + God of Thunder and Storm. (Thor, Taranis, Zeus, Indra, Perkunas etc...)

No doubt Gods evolved, but reflexes demonstrate certain aspects of God remained fairly static. I have theories as to why that would be the case, however that would take to long to cover in a comment.

Nobody is going to take you seriosuly if it's just, "Here's my theory without citing any evidence." In contrast to your claim, here's how the Oxford Introduction to Proto-Indo-European begins their section on "Deities":

Below are summarized the names or types of deities that have generally been reconstructed to Proto-Indo-European as we have seen in Section 23.1. It should be emphasized that the proto-categories are perhaps more abstract that their single name might suggest, i.e. what is meant by a war god may have actually included a number of different deities within the earlier system. In some cases we may find the same individual under two different names, e.g. ‘sky daughter’ and ‘dawn’ would appear to be the same deity. In other cases, a single deity from one of the Indo-European traditions may be included under a number of different headings. Just as a screenwriter when working from a literary source will routinely collapse different characters into a single individual to have a manageable cast for his script, so also did the different Indo-European groups juggle with their deities to fill out sometimes multiple roles, e.g. the use of the sky god in Greece and Rome to fulfill roles of the thunder god, war god, and others.

In other words, the gods weren't static. They sometimes did retain some main attributes, but even the sky god (who would be known to the Greeks and Latins as Zeus and Jupiter) underwent significant change. How could they not? There were 3000 years, or so, between PIE culture and the ancient Greeks. The Romans were even later than that.

Greeks were fairly smart people. Take Plato's account of the myth of Phaeton: https://en.wikipedia.org/wiki/Phaethon#Plato's_Timaeus Not all Greeks may have believed in the literal existence of the Gods. - So they referred to the "religion/cult" of a God as the God themselves.

I don't understand what the importance of this statement is. Nobody ever claimed all Greeks believed the myths. The only claim is that myths evolve over time. It's hard to keep stories straight over millenia, and a good storyteller may add new embellishments that become part of the canon -- until someone else comes along and changes it again, as the Oxford book I cited explains.

Hence my research.

How do we expect to find cognates across multiple PIE cultures if the etymology itself is so poorly attested?

Find the etymology and the PIE Hermes will emerge.

Your research is mostly just Wikipedia and Wiktionary entries. That's not real research. At best, it's background to start doing research. I would be careful, because, just like you, some Wiki editors may have their own pet theories and scultped those Wiki entries to match them. You should be confident enough in the literature, as well as the primary and secondary sources, that you shouldn't need Wiki to do this, if you want this theory of yours to go anywhere.

As per my research, and aforementioned comments of Pan/Pushan, it may be possible that there were two competing PIE cults of very similar nature - Pushan and Hermes/Mithra.

A lot of assumptions with still no evidence, aside from trying to tie together some Wiki entries.

Oh, he certainly was a pre Roman God, it's in both the Wikipedia page and in the very same Oxford Classical Dictionary you just shared:

"Traditionally the Terminus on the Capitol had been there before the temple of Jupiter". Wikipedia explains that he was a pre-roman God that was assimilated into Jupiter.

​That temple was still within Roman times, as the Oxford book explains. The Wiki entry says that he was "of Sabine origin", which was a culture on the Italian peninsula contemporary with the Romans. I think you still need to find a source that claims Terminus dates back to pre-Italic, because the sources we've looked at say he doesn't.

This right here... I do not understand AT ALL why I can't find a research paper explaining this. Without an explanation it feels like it is laziness. Almost like "God of the Gaps" but for etymology. I've already explained to you that the origin of this is in the book This right here... I do not understand AT ALL why I can't find a research paper explaining this. Without an explanation it feels like it is laziness. Almost like "God of the Gaps" but for etymology.

I already pointed out that the source for this is the later editions of Walde-Hoffmann's book, which aren't online. Have you engaged with this book yet? It occurs to me that we're the lazy ones for not actually looking at the scholarship that's not always digitized. (Interlibrary loan is your friend.)

But even then, there's no evidence that Merx came from Etruscan, it is only speculated to do so. - No word we have ever found in Etruscan provides evidence for this, Nor is there an Etruscan God called anywhere near the word "Mercurio".

There's no evidence it came from anywhere. That's the rub of the whole thing. It's enticing to look at merx and mark in Latin and think "Ah ha! They must ultimately have the same root!" Yet, language is full of false friends. Without evidence, it's not even a theory. Just a hypothesis.

The reason that the Etruscan hypothesis has legs is that we don't see 'merx' anywhere else in any PIE language. If it's of PIE origin, where is it hiding in other PIE languages? Even if merx and mark do have the same root, then how does Latin end up with two separate words when all other PIE languages before it had only one?

A reasonable hiding place is that it doesn't have a PIE origin. And if that's the case, then who are the likely non-PIE suspects to come in contact with the Romans? The Etruscans are top of the list. That offers a reasonable explanation on how Latin came to have both merx and mark while all other IE languages before them just had mark.

Now, that doesn't mean that the Etruscans couldn't have borrowed it from an earlier PIE culture, tranformed it, and then the Romans took it back from them, and that's how they got two words. Yet, if that's the case, and we're wildly speculating about how this could have happened, then a godly origin isn't necessary. The Etruscans could have learned the PIE mark meaning boundary/border, because it was the term used for the border town they did trade in. To them, then, this term morphed into merx meaning market. And then Latin ended up with merx = market and mark = border/mark. A pretty simple and reasonable explanation, which doesn't require an unattested adoption of religious deities - it mostly just requires the words themselves. Yet, just like any other explanation, there is no evidence for it. So what do Latin scholars make of all of it? They just say that merx is probably not of immediate IE origin, which would mean its most likely origin is non-IE, like Etruscan, but unconfirmed. Since it's unattested, they leave it at that as the best we can say given the currently available sources.

I don't think your theory is a bad one. I think you probably could make a decent case for it in an academic paper, if you actually engaged with the scholarship first and learned all about it. But this latest research doesn't convince me any more than your original post - it's just got more Wikipedia/Wiktionary links now. Once you actually have a good working knowledge of the subjects of PIE reconstructed mythology, as well as Latin, Greek, and Etruscan etymology, you can try to debunk it from a place of knowledge.

If you aren't motivated, one thing you might try is to contact the authors of this paper. It's a recent paper where they appear to try to reconstruct the roots of merx and Mercury into Oscan. Brian D. Joseph is a linguistics professor at Ohio State, and Rex E. Wallace is a professor at UMass. You might try emailing them and asking about any previous scholarship related to the Latin etymology of merx. I wouldn't bother them with your full-fledged theory, but even a simple question like, "Why do Latin scholars think merx and mark came from two separate origins?" may lead you to some of the academic basis for this claim.

Bottom line, from our thread, I am still unconvinced that either one of us yet has the background knowledge on the necessary subjects to make these claims without running into some scholarship (Hoffmann?) that we haven't read, which will explain why this theory has been considered and rejected.