It is working!!
Thank you all very much, those who helped!
I note the details below, on the offchance that it might help someone else. Of course, the ideal is to avoid the problem in the first place by *unplugging every other drive* when installing Windows.
( Along the way I got into my machine by [as per the relevant step in the setup precedure] launching Clover from my setup USB stick and selecting my MacOS partition... but I am not thinking that that made any difference. (
I tried to reinstall the motherboard graphics driver et al., but that failed (Console: "~/Library/Logs" : MultiBeast.log : "Certificate used to sign package is not trusted." [It worked in 2018.]).) )
(During startup, it spewed out several pages of some message ending in "
[...] :1", when it was loading my drives. {That} I have never seen before, but it does not seem to matter... .)
As I said, I replaced the latest Clover version/ EFI setup/ config.plist with an older one that I thought should work.
* It is using VBoxHfs-64.efi.
* AppleKeyAggregator-64.efi still returns an error if I try to load it manually in the Clover Shell. [Note to beginners: start with "help -b" and work your way up from there. Change to volume: e.g. "FS0:" or "BLK6:". "cd ..."
[standard "change directory"] and "ls" work.]
* I have changed from OsxAptioFix2Drv-64.efi to AptioMemoryFix.efi ; this is the only difference from how it used to be, I think.
* Informed byUtterDisbelief's comment about graphics (...
Well that last line usually points to a GPU problem. So either at the switch over from boot graphics to main system, or a bad configuration depending on which GPU you are using.
...) -- as a stab in the dark -- I switched my NVidia setting from "OS X Default..." to "NVidia Web...", and turned on the pertinent toggle in config.plist/Clover startup.
It appears that the latter is what made the difference, pertinent to that.
(I had earlier changed from OsxAptioFix2Drv-64.efi to AptioMemoryFix; that or reverting to my Clover 4173 setup got me past the failing after the "++++...+++" at the outset.)
[It has all been rather stressful, as I did not want to make any changes in a setup that could reasonably be expected to be fine. Conversely, I am thinking that Clover v5150 was unexpected, and that I was used to seeing v4173... and that the problem arose (*some*how) from my deleting the "other" EFI partitions. That aspect will remain a mystery.]
Anyway, thanks again!
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... Actually, one last question, if anyone cares to engage with it. My audio works on about 2/3s of my reboots, and the machine does not reliably sleep. Might updating Clover (with any attendant changes, such as changing drivers64UEFI to drivers/UEFI) help with any of that, please?