Bingo! The culprit was found. The Northbridge.
The original cooler on it was a brass plate, connected with a Perkins pipe to a large heatsink, sitting on CPU VREG MOSFETS. Since I am using water cooling, I removed the plate and the tube, and installed a water heat exchanges onto the chip. There was a layer of silicon(?) film (of the color that I'd describe as the darkest beige) in the original assembly, that I kept. That shown to have been the wrong move made; it is also possible that I tightened my assembly up to a pressure that was too low for this type of medium.
All in all, after I replaced the liner with some metal-laden paste, the system easily booted up at 395 MHz with all the same voltages except for Vcore = 1.400 V, showing Tcore=49C and Twater=33C at ambient 22C, although I had to back up to 380 MHz for stability reasons. This by far transcend my initial goal of a completely stable system at 350 MHz, so I am there.
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CPU-Z 1.39 shows Vcore=1.213V constantly when it's higher than 1.45V.
Apparently, the limit depends on make of motherboard. I experimented lowering Vcore at low Fclk down to system instability, but never registered anything but 1.213V displayed. Although I did not directly meter the voltage, I noted a rising of Tcore when upping Vcore in the setup, which should serve as an indirect evidence of real Vcore growth. Nothing of that suggests that Vcore was less than 1.45V, of course, but if it was this high at the BIOS setting of 1.200V, then my top voltage test of 1.5V setting would perhaps evaporate the CPU, unless we assume that the dependence of "real" vs. "set" Vcore is highly non-linear, and that I hit its "sweet spot" when noted Tcore following the BIOS setting. An assumption that CPU-Z does not fetch the voltage correctly on this board sounds much more reasonably to me.
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[…]running non-optimized OS with default MICROSOFT drivers is like driving the cab from passengers' seat, when MICROSOFT is your cab driver. All you can do, is tell him where to go and HOPE he'll get you there ALIVE
You are right, to a degree; consider, however, that there were billions of Windows installation in the world, and how complex it is to write software that works in such a huge variety of situations this number alone prompts, hang-up upon installation being a rare exception. Not to prove anything, but consider my software experience during this system installation: ABIT own control program leaked 1,500,000 handles already by the time when I found out "what was wrong" with the system; Intel Matrix Storage manager complained that the system was below minimum requirements for it (BS), and I was fortunate to recover only the driver from the package; Intel thermal test did not start up (unfortunately, your suggestion did not work, as did not any one of sensible BIOS settings affect it); ZCPU and SpeedFan installed and kind of worked, with reservations for the wrong Vcore indication in ZCPU, and the fact that SpeedFan displayed only 2×Tcore and 2×HD temperatures, no voltages); Intel chipset INFs installed, but killed all USB root hub drivers, requiring their manual reinstallation. Microsoft software looks sweet on this background, for it required no significant intervention and did its job.
All this being a matter of personal preference, if I were to choose whom should I put into the driver seat of your cab example among Microsoft, Intel, ABIT and the two individual authors of respective ZCPU and SpeedFan, then certainly I would have chosen Microsoft. I am fully agree with you that Microsoft makes imperfect software, but alternatives, when exist, are usually still even further from the perfection unattainable…
And thank you for your replies: in the area such "unofficial" as overclocking, any piece of information is self-valuable, and I do appreciate your time and effort on sharing it.
Edit: s/ZCPU/CPU-Z/