I'll have to take your word for it if you say you haven't found it on Perugia Shock; I'm pretty sure, though, that Frank does talk about Battistelli's initial report as one reason he may have stuck to his various dubious claims - perhaps it was in relation to entering the room, rather than the arrival time. At any rate, I'm glad someone has the posts archived; hopefully you or Frank himself will be able to make them available again at some point.
As Kaosium said, the first mention of the 12:35 arrival time seems to be in Matteini's report, and it's worth noting that there's no reference there to the CCTV footage at all (or not that I could see, though I should point out that I have the unsearchable pdf version!). So either Matteini is basing that 12:35 time on some kind of information from the postal police - e.g. their logged arrival time, or a report - or she's basing it on an estimated and altered CCTV arrival time, but without mentioning that. I tend to think the former is more likely simply because there must have been a report from the postal police, presumably shortly after the murder, and arrival time would be a basic piece of information to have on there.
Then there's Micheli's report: here's the key sentence from the part you quoted:
I'm not sure what's meant by a 'rounded' hour, perhaps it just means an approximate time (obviously we know it isn't actually 'rounded off' to the nearest 5 minutes or whatever). But the key point here is that Micheli clearly refers to two sources of information: Battistelli's written record that they arrived at 12:35, and then the camera footage which possibly even shows them arriving earlier than that.
So in both Matteini and Micheli's report, that unambiguous 12:35 arrival time is given. To me, the simplest explanation for that time would be that it came from the postal police report/log. It's possible of course that they scrabbled around after the fact trying to reconstruct what happened, but that seems quite convoluted when all they needed to do was estimate the time they arrived. I doubt Battistelli had any idea how important that time would turn out to be.