Downgrading wont work after 2.10. It is assumed that there is either a version flag or a revocation field inside syscon (or another writeable area) which prevent from downgrading.
My personal theory is that there is a revocation area inside syscon (most likely some certificate) which is updated everytime you update and will prevent an old coreos from running unless it is not added in the revocation certificate (which should be the case of debug core os and ones used for servicing purposes, though I expect all the ones from retail updates to be added)
To cut the cost of PS3 and make it more profitable they take out a few features, that they think the regular consumers don't don't and add more mainstream feature that are software driven.
One thing i dont get is why the ps3 slim is able to play dvd normal dvds, so doesn't it mean that it can play ps2 games or if not that backups.
AFAIK only the original PS3 60Gb could play PS2 games - the other PS3 phat models (40Gb, 80Gb etc) cannot play PS2 games.
It seems the PS2 emulation was hardware based as the 60Gb also had 4 USB and a flash memory reader which is different from the later models.