Ok, here is the deal. I was reading somewhere about TTY bolt reuse tolerances. Overstretching makes the bolts thinner. Checked my bolts again and found two which have uneven thickness. In particular. All other bolts having 8.90mm thickness throughout the thin part, but those two are going down to 8.82mm. Moreover, these bolts were behind cylinder 2 where cooling is not so good i think. The formula seems simple. Cooling error made this part to overheat and stretched these bolts, also resulting in coolant leak behind cylinder 2. Losing even more coolant made it worse. Now the question. What if the leaking radiator was first making coolant level low, possibly resulting in an air bubble behind cylinder 2 under the gasket? Is that possible?
Replacing these two bolts only seems to be a safe and affordable way to go.
Ok, another look at the manual. "If any of the threads or bolt head areas are damaged, replace the bolt(s)". Very confusing but as i understand this means bolt OR bolts.
Another comment. The threads and bolt heads are sound, only the yield part has a flaw. So, according the manual i could use even these again. Here is a picture of the mark at the thin part. Look for the belt around the bolt yield section.