Ok, quite a few steps forward. I did check the CPU cooler (thanks again, Pastrychef) and it was too tightly screwed down. But that didn't change things, except that when I moved one memory piece fro the recommended A2 to slot A1, it was recognised. Eventually I got both sticks working in their recommended Gene slots, A2 and B2. Inserting the RAM properly obviously requires great care and experience obviously helps.
The Gene BIOS setup was quite daunting. Thanks to Stork's guide, which tells us in which BIOS menu to find things, I got that done. The BIOS version 3401 I did the update to is there OK.
Gene is obviously very picky about which USB ports work for various functions. Stork pointed that out, but there seem to be changes. I mentioned before the change in the port for the BIOS Flashback port, which is stated wrongly (for my USB, anyway) on p. 2-12 of their manual and correctly on p. 2-13! Go figure! I found similar pickiness about which port the USB installer was recognised in. It did finally work in bottom USB port below the PS2 port, as Stork has said. Also, My AOC monitor wouldn't display anything with an HDMI cable, but DVI worked.
Status at this stage: I can get to the Clover boot screen, select any options (verbose, etc), and launch it. But then it immediately hangs with the Apple logo showing. My USB installer uses macOs 10.12.5. If it means anything useful the Gene Q code when it hangs is 61 = "NVRAM initialisation". A Clover verbose setting shows only this:
OsxAptioFix2Drv: Starting overrides for \System\Library\CoreServices\boot.efi
Using relic blocks no. hibernate wake: no
I've had a crash refresher course in how detailed and specific the settings have to be, so that's enough for today. I would be very grateful for any advice.