Mmmm - OP - I really would not press the issue, 12 is still awfully young & she is probably horribly embarrassed & just wants you to stop talking about it.
I think, in most respects, these things are better left to the peer group - educate her about the choices, let her chat to her friends about it & leave it up to her.
& Herbex, the first time I used tampons, at 15 & a virgin I couldn't get the chuffing thing out. I pulled & pulled to no avail until, in panic I got a mirror between my legs & there was a piece of my nymen stretched across the end of the tampon, it was horrible, I eventually cut it with the nail scissors as that seemed a better option than telling my mother & letting her have a look. I didn't use them again until about two years after I lost my virginity.
Aaaannnd when I did lose my virginity with my first boyfriend my hymen still didn't break and consequently hung down & was painful & swollen until we had had sex a few more times, so maybe it wasn't a membrane at the opening of my vagina but it was certainly present. Other than that I don't understand your point.
As for teenage girls leaving bloodstained knickers around, I used to hide mine in the cupboard in my bedroom. The cleaner would always lay them out in a nice, neat pile. She may have been making a point.....