As far as I know all of the latest mask 060's can handle at least 100Mhz, the issue is what you attach it to (ie the process board)
No.
68060 rev6 have 3 sub-revisions : -002, -004 and -006
Some -002 and -004 cannot reach 100 Mhz, don't know why exactly...
The best is the -006, I have two of them and they are working fine at 105 Mhz...