I was going to object to the poster's math based on how
chapter 28 was (mis)translated as the weirdly-specific 200,860,200 - but that's apparently why they linked the raw. I transcribed the raw to Google Translate with the help of the furigana and the handwriting tool, and 2億6千2百80万回分前のお話 does come out to 262,800,000 heartbeats.
They apparently assume she's only nine years old exactly when the flashback starts, but none of the context suggests it. Given this and the fact we see her heart rate as being in the 50s when she's awake (it slows during deep sleep), she might actually be 18-and-change when she starts going out with Kazuya (the post deduces 17-and-change).
Regardless, on her birthday (
chapter 139) she's presumably either 18 or 19. So in the US at least, she's able to consent.
Something I picked up on while looking for info on the above: When Ruka proposes a trip to an onsen (
chapter 169), Kazuya objects that she's "underage". Iirc, in Japan you're an adult at 20 - but I don't whether that's only wrt being able to stay in a hotel, or wrt their age-of-consent laws as well.