Disclaimer:

Most of this info is spread throughout the "Rogue DPS Spreadsheet"; this is for easier reference only.


Results:

1) all extra swings deal main-hand damage, modelled as a yellow attack (no hit-penalty, no glancing blows)

2a) extra swings caused by main-hand autoattacks trigger poisons and other mainhand enchants

2b) extra swings caused by off-hand autoattacks trigger Combat Potency

2c) extra swings caused by abilities (this includes Shiv) don't trigger poisons, enchants or Combat Potency


Testing scheme:

2a) Single-wielding a slow main-hand, only autoattack, repeating until a number of double poison (tested with Wound Poison) or double enchant procs (tested with Fiery Weapon), or both occurred. In all cases, the previous and next autoattack was logged more than 2 seconds before or after, which makes interference very unlikely (no double procs ever occurred without a preceding extra attack, in about an hour of testing)

2b) Wielding a sword only in off-hand, repeating until a number of double Combat Potency procs have occurred. In all cases, the previous and next autoattack was logged more than 1 second before or after, and as with 2a), no double Combat Potency procs ever occurred without a preceding extra attack; about 2 hours of testing. In all that time, not a single double poison or enchant proc was recorded, with poison applied to either main- or offhand.

2c) No double procs of any kind were reported in over 2 hours of testing (turning away from the mob until no more white attacks took place)