However, the typical estimates of transmission rate are extremely low: 0.05% / 0.1% per insertive/receptive P/V sex act 0.065% / 0.5% per insertive/receptive P/A sex act

At an (unrealistically?) independent 0.5% chance per act, a 50% chance of transmission would require 139 sex acts — hardly "years and years".

That is for the highest transmission activity - receptive A, so be careful not to cherry pick. Yes - 139 unprotected sex acts. It would take 1390 unprotected insertive A sex acts to reach a 50% chance of transmission.

So with some assumptions, mainly - gay bathouses and no condom use - yes the virus could spread horizontally, in theory. Although that population would necessarily first acquire every other STD known to man, more or less.

But in the general heterosexual population, not a chance. If you compare to the odds of pregnancy from unprotected sex, the insane requisite amounts of unprotected sex with strangers would result in a massive baby epidemic and far more vertical transmissions long before it could ever spread horizontally in the hetero population.

I don't know why you mention "modern developed-world rates" and then have a link to 1901 NY and Africa . . .

So in my mind this makes it technically impossible for HIV to be an STD.

At best, this can show that pandemic AIDS can't primarily result from sexual transmission of HIV, which is evidence that AIDS has causes other than HIV, but also that pandemic AIDS spreads through other means (as suggested here.

You don't need the "at best" qualifier, but yes I agree that is what this shows. Showing that however opens a crack in the entire facade. Perhaps not a critical failure, but a significant doubt nonetheless.

If the orthodox position had updated on the evidence, and instead changed their claim to "HIV is a borderline infectious disease that spreads primarily through the prenatal and blood-borne routes", then I would give them more creedence. Of course, for political reasons alone they could never admit that.