My focus on the DIMMs themselves arises out of the fact the "memory test" portion of the boot process takes place in firmware and once the alarm is raised, the boot process stops. The OS itself can't be loaded and therefore cannot be identified or tested so the Mac has no way of knowing what OS is installed.
I'm still hanging my hat on your item 4: "A DIMM module performing below or at the lower end of the specification range (replace the DIMM)" Even if whatever arcane diagnostic routine a machine goes through pre-boot can't determine which version of macOS it's being asked to boot, can't it still determine that whichever version it's being asked to boot is beyond the DIMMs' capability?
From the sequence of events, it occurs to me your DIMMs might have been subject to ESD (Electro-Static Discharge) damage. It has happened to me in the past. All DIMMs are vulnerable and strict anti-static measures should be followed to prevent their being
fried in handling. Testing using your daughter's DIMMs is a good idea, but under the circumstances, I would review
anti-static handling procedures and follow them assiduously to be sure her DIMMs didn't get
fried as well. (Now the questions are: "where did I put that anti-static mat and wristband, and do I still have any anti-static spray?"
)
I've worked on HDDs, SSDs, and DIMMs any number of times without having fried anything, which, of course, is no guarantee that I didn't do so this time, but I did handle the 8 GB DIMMs immediately after the 4 GBs "beeped," and I had no problem with them. (I'm sure there's some way to test DIMMs definitively, but I'm equally sure that the means to do it are beyond my ability to access.)
And while we're scratching our heads, here's an interesting question: Assuming that the DIMMs haven't been fried - which I am - why did the machine boot with them before I changed the HDD, but not afterwards?
I wonder if this qualifies as a "unified" theory of all that was wrong with Rita's MBP: I've already established - by both booting from my external and deleting a large chunk of data from the MBP's internal - that "offloading" "load" from the HDD improved performance, so I'll extrapolate from there and guess that the debilitated HDD diminished the OS's performance threshold by enough to raise it above the DIMM's performance threshold...self-cancelling errors, so to speak.
And I think I'm going to copyright that as a pretzel recipe!
Where is V1 when we need him? Too bad he doesn't check in any more. (It would be nice to give him - and let him know he's got - his own forum, same as MMT3 used to have, so he can check in periodically and immediately see if we could use his help.)
