OK fine. We have easily-identified DNA sequences, some of which are in the same positions in the genomes of different animals. What can we say about which animals? The observed facts are the sort I've been stating "IF a genetic marker is present in chimps and orangutans, it will also be found in people and gorillas", etc. This is a tree structure.
Note that so far nothing but genetic sequencing has entered the picture.
You assume common descent the instant you start 'tracing' a phylogenetic 'tree'
You're a bit confused here. The tree obtained from the ERVs is compared to the preexisting phylogenetic tree, and found to be the same.
You have not shown that ERV infections happen only once and then move to fixation in any species,
No, that's a deduction from the fact that all the ERVs follow the same pattern of presence/absence, that just happens to mimic the phylogenetic tree. Also, consider the Asian apes. An ERV common to both of them is inevitably found in all African apes. This argues against independent infections in many species.
... much less all of them. You merely assume such. That much is clear.
I don't understand what you're trying to say here. The theory is that the ERV got fixed once, in a common ancestor.
You don't even know that these are ERV's, you assume that because you think you find 'pieces' of a viral genome. You don't actually find a genome, you impose it on the data.
They just happen to be very much like what present-day viruses do to genomes. Therefore the correct phrase would be "find it in the data", not "impose it on the data". But, maybe, that's wrong. You still have genetic data that exactly matches the independently-derived phylogenetic tree.
There isn't just one ERV. There are thousands of them, in humans. Your infection-to-fixation-to-presrvation scenario must be played out thousands of times to produce the pattern you see. Not possible.
And you can't say that your data 'exactly matches the phylogenetic tree'. The 'tree' doesn't exist. The 'forks' are all imaginary and all of the bushy leaves haven't been sequenced.
You are way over-extended.