To be honest, I'm struggling to understand your point. Especially considering you are making a lot of - to the best of my knowledge - questionable assertions.
Hell, I could summarize my objection as "are you sure it wasn't power difference, courtesy of technological advancement, between the colonists and indigenous populations that drove the interactions into the historical 'you have I want and you get no say in it'" paradigm?
Myeah... except most of the deadliest historical diseases are established to have emerged in humans (and often are not capable of cross-species transmission). I think measles is the exception in that it has animal origin, and tuberculosis is supposed to have propagated cross-continent through animals, but that's pretty much the exception rather than norm based on the historical information that we do have.
It's only pretty recently that we have had a whole host of these crop up (like HIV, the older SARS and now COVID), and it may very well be related to the basic premise of epidemiology - the more densly populated a sample area, the more likely you're to see emergence and spread of any new pathogen, whatever its origin.
Maybe, maybe not. If Runey's elves lived in isolated communities (which was mostly the case for Native American tribes), even if some deadly new disease emerged it may not have had the opportunity to spread to be present in their population. Again, basic rule of epidemiology - the fewer people around, and the greater the distance between their societal clusters, the less likely you're to see anything deadly emerge in the first place, or successfully hang around.
Also, I'm pretty sure the argument over whether syphilis originated in Americas or not hasn't been concluded, and the only thing we do know is that the age of colonization spurred its mass-spread in Ye Olde World - and that those societies had greater capacity to absorb any losses from it and continue their advancement than any less-developed tribal culture.
Maybe I'm missing some point here, but on its own this claim is laughably false. European colonists, despite their lower numbers (this goes doubly for Cortes and his fellows, who mostly relied on indigenuous tribal 'allies' - that they frequently screwed over right after anyway - in their conquests and themselves constituted only the "Schwerpunkt force"), were capable of leveraging their techological advantage (along with incomparably more superior support base in the shape of Old World countries that always could replace manpower and materiel losses) to overpower any attempts self-determination of the natives in face of the invading foreigners.
And yet again I'll point you to basically the whole world (not just the Americas) as an example of indigenuous diseases doing diddly to stop European colonialism. In situations where cross-species transmission isn't even a requirement, though the fact that in Runey's universe interbreeding is possible basically makes it clear humans and elves aren't even - biologically speaking - distinct species. Unless it's magic, or just something Runey didn't really think too much about
Obviously disagree, and I think your analysis is flawed not to support that conclusion even assuming a lot of lenience toward personal interpretation of Runey's intended world-building.
Edit: You know... it just struck me that, hundreds and hundreds of pages of overanalysis - in this thread alone - later, Runey is probably sitting somewhere in the corner going, "I just wanted to make a porn game, man..."