1) +DMG Augments I think apply to all hits, regular or WS. You might be thinking of the augment from old Skirmish +2 weapons, which was "Weaponskill:DMG:+%".
2) WS Damage% only applies to first hit.
3) Offhand will count as an extra hit unless you've hit 8 swings from multi-attacks already.
4) Multi-attacks can only activate once on WS, and I recall reading only on the first hit.
5) Multi-attacks are only really useful on WSs that carry fTP (common on high-level multihit WSs). Crit Rate is useless unless the WS can crit (like Evisceration, CDC). Crit Damage is useless unless the WS can crit or you have SA/TA/Mighty Strikes active.
6) Yes, SA and TA are best for WS's with a strong main hit, like Rudra's or Mandalic Stab.
7) Yes, Belt/Gorget are best for those WSs, even more so if they are multihits.
8) I imagine RNG's crit bonus would apply to Jishnu's Radiance, since that's a crit WS.