Gidday there Homer,
Welcome to ECN, mate!.
I must agree with the majority of your comments.
However, one thing has to be said about this cable and it's voltage rating.
Now current ratings are one thing, voltage ratings are another.
Here in New Zealand cables used for 230 or 400V are rated at 600/1000V, that is 600V between cores in the cable and 1000V to earth.
However I would agree with Bob (Iwire), it sounds like you might have got a bad batch.
Moisture in a cable should not cause a problem like this unless there has been poor sealing of the cable and it's cores in past repairs.
Which brings me to another thing, adhesive lined heavy-walled heatshrink is the only thing allowed here at work for jointing cables these days.