Hi Zip! Thanks a lot for the detailed response and your kind words!!

• Pressure testing the radiator is a very good advice that I didn't thought about (I just did a visual inspection of the "new"-used- radiator, and a water flush, checking if it was somehow watertight, and called it a day),
In fact I can recall pressurizing the system it's described as a test for cooling leaks in the service manual.

I'll try to ask in a shop, alas being august (normally all of them 'round here are closed due to vacation, the few that aren't, are up to their ears with work).
Either that or buying myself a pressure tester for radiators/cooling system, those with a hand pump and a set of cap adaptors.

• In regard to the possibility of "bad omen oil sludge", that's the very first thing I checked when being told about the exploded radiator. I looked to the cap and oil stick too, both seemed clear, foam or sludge free, and when I drained the cooling lines and radiator prior to substitution, there wasn't any oil neither sludge in the refrigerant.
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When I have the radiator installed, I'll check the system with a head gasket CO2 chemical detector that I have, those with a little 2 chamber ampoule and the blue liquid reactive.

Hopefully may not be a head gasket leak, I wish Lemmy hear my prayers (being from Motorhead... sorry couldn't avoid the bad joke)

• About the white smoke, I may not explained myself correctly, the steam came from the motor compartment (due to the ruptured lateral of the radiator thus provoking a massive leak), NOT the exhaust tail. In fact, I don't remember seeing white smoke out the exhaust when I started the car for getting it off the tow truck.But, having dismantled already filter box, fan, old radiator...

I'll grab the opportunity to take out sparkplugs and look for an abnormally clean cylinder/sparkplug (I have a little smartphone borescope, bad quality image but enough to tell if the cylinder head is black or clean white).
Also I'll do a compression test in all the cylinders, or pressure testing with compressed air as stated in the service manual, looking for bubbling in the reservoir.

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• I'll follow straight away your tip about a new reservoir cap, certainly it may also be an additional cause for the blow up, not opening and releasing pressure when it should.
In fact, the reservoir area was dry, thus I can conclude if there was any pressure, the cap didn't release any drop of it.

• Last but not least many thanks for the detailed breakdown about the working temperatures and the oil coolers!!
Certainly, I didn't thought about the difference in working pressures, either different densities between ATF and motor oil. According to service manual, the motor oil pressure at 3000rpm could be in-between 45-105psi, so don't knowing how much pressure that particular radiator transmission circuit could handle, and considering as you pointed that wouldn't do much in terms of cooling down the motor oil, I'll just plug the holes untill my next cooling whacky idea

• BTW for whoever reading this, these transmission oil holes are M14 in diameter, but 1 or 1.5mm thread, I know because I bought regular M14 bolts for the task, I believe 2mm thread, and can screw them just 2 turns until the point it would start crossthreading the hole, but for my purpose that's only to close the holes preventing dirt to get in, that's suffucient. I thought about printing plugs with the 3D printer, but I haven't ABS, just PETG that has its glass transition point/ loss of strength at ą85ēC/185F, so it wouldn't be a feasible or reliable option for the radiator.

Thanks for the help, I'll update whenever possible.
Red.