Well, the study of exhaust pulses is a career in itself for sure. So, don’t spend a whole lot of time trying to make sense of things that seem so unlikely. Just know that negative exhaust pulses can attract water from the most unlikely sources. And, it is surely possible to have just one cylinder "attract" the cooling leak, and that white mess is consistent with the exhaust/water spray in the manifold. You have found some holes in the inner exhaust cover, which to me, explains both problems.
Unfortunately, it surely is possible that there are other pinhole water leaks, casting flaws, or corrosion problems adding to the leakage problem caused by the leaky inner cover. I suppose you could rig up some sort of plug to block up the water discharge from the block, then pressure test the crankcase. You would have to be very careful not to over pressurize the block overcoming the gaskets/sealer ability to seal.
Again, this is a $20 engine. I’m thinking the inner exhaust cover is all that is wrong here, but I would surely give the block a close visual inspection prior to installing the new inner cover.
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