After posting and thinking about it I realized it would be actually pretty easy to test the double damage thing with the Untainted mod (it tells you your largest hit at the end). So I can confirm now that it does indeed double your attack damage, not just basic attacks.

The only problem I got with this node, is that it doesn't work on spell damage, too. It's perfectly fine otherwise.

15% chance to do double damage is effectively a 15% multiplicative damage increase. That's good, but not good enough to be overpowered or anything. There aren't too many damage increases like this on the passive tree, and there needs to be a few such nodes, IMO.

Edit - I didn't even notice that Probing Weakness actually increased your resists, lmao. But yeah, that should obviously be fixed.