I realize this is now an exercise in mental masturbation (not to be crude...) but in case anyone runs into the same problem as I. I got Yosemite installed and booted, and yes, all my SATA drives mount just fine now.
My issues were, in order of repair:
• achieve true UEFI clover boot (had to start with a very clean, sparse config file; used my DSDT that was pre-edited from a Mavericks/Chameleon system, and turned off Clover's DSDT fixes; copied cloverx64.efi to the root directory and named it shellx64.efi, though I don't know if that was required...)
• get kexts injected properly (fakeSMC, NullCPU, Ethernet) by adding <key>injectkexts</key><true/> to config file
• resolve USB problems (plugged USB flash drive into first USB port, turned off EHCI handoff in BIOS, injected genericusbxhci.kext into Clover's 10.10 kext folder... unknown if needed, but my USB issues while booting resolved themselves after these steps)
Outstanding issues (but not deal breakers):
• getting UEFI USB boot to "stick" as my primary boot option... it keeps reverting to legacy, and I have to go to the boot manager to select UEFI... :-/
• I also still haven't used migration assistant to get my old install into the clean Yosemite install. That might screw **** up.
I have CCC'ed the clean good Yosemite install to an image, so I can always start fresh quickly if I need.
Thanks for listening. It was cathartic.