1/ As mentioned above, there is metldr.2. There is also lv0.2 and possibly (hasn't been confirmed but I have no doubt they have that revision on 3.60+ factory units) bootldr.2.
Metldr.2 prevents loading 3.55 CFW because they removed its ability to decrypt lv1, lv2, isoldr, rvkldr and appldr since in 3.60+ lv0 does that.
lv0.2 prevents loading a modified lv0 (should the keys ever be released), supposedly it's a metadata for lv0.2 encrypted with new, uncrackable keys, and bootldr.2 would check lv0.2 and compare with lv0 to make sure it hasn't been modified.
The only way to get CFW running on those newer models is finding a completely new exploit (so starting from scratch).
2/ Just because someone has 3.60+ lv1, lv2, appldr and npdrm keys does not mean they have lv0 keys. They could have an exploit of a higher level than lv0.
PS3 Slim CECH-3004A 160GB (500GB)
PS2 Slim SCPH-70004 - FMCB 1.8b