The problem with memory speed turned out to be more complicated. While IMC and DDR3 voltage had to be increased slightly to stabilize the system, another setting (again on 'auto') caused many other problems.
DDR2 and 3 specs require some component(s) that automatically detect and adjust various drive strength values so that a board that supports DDR2 or 3 can be compatible with memory modules of matching spec. However, many board makers have cut costs by cutting corners: these component(s) are left off the boards, and in their place: QVL, tons of BIOS updates, and tech support inquiries all contributing to big compatibility tables. When a module's not in the table, the BIOS just makes a guess and whether it works or not comes down to sheer dumb luck, even if both the board and modules are otherwise perfectly functional.
Changing CKE Drive Strength from auto (2.0x) to 1.5x made a night and day difference in stability at the modules' rated speed and timings. I literally could not boot the system without errors and crashes at 1600 MHz with the 'auto' setting (2.0x), yet manually setting it to 1.5x allowed me to boot error-free all week at 1600 MHz. Even down at 1066 MHz, the memory still had occasional problems with the 'auto' setting. With the BIOS settings adjusted, so many annoying little issues are gone. The clipboard doesn't stop working at random times. I don't see certain occasional errors or crashes in various apps. My network connection is more stable (due I assume to the packets or related data not being corrupted).
Needless to say I will not be buying another Asus board. (They would have to give me 4-5 free boards to cover the cost and headaches, in time spent troubleshooting, just to break even.) I will make certain the next board either meets the memory spec it advertises (e.g. DDR3) or swallow the lie by only buying from the list of 'pre-qualified' modules with which it is actually compatible. Hopefully the modules will be at the same revision: again it comes down to sheer dumb luck, as memory manufacturers ship often new batches of same model with different SPD settings and/or electrical components.
Before someone makes the (obnoxious) assumption by advising me to not buy cheap ram, I have (separately) tried Corsair and Kingston ECC in my system, and each set of modules worked fine during extensive memtest runs in another system. And yes I know about testing them one at a time, testing them in dual channel, looking for bad slots on the board, not mixing speeds/timings/brands/revisions etc. Drive strengths were adjusted only as a last resort, something tried after all other normal troubleshooting failed to completely address the issues.