does double/triple/quadruple (D/T/Q) attack from traits/gear/abilitys affect WSs that is a single hit but is counted as a multi teir dmg WS?
E.G. Blade:Hi - Deals quadruple damage
E.G. Rudra's storm - Deals quintuple damage
if D/T/Q attack does affect WSs like these, is the extra hits calculated before or after the ftp?
and is it more worth to aim for the D/T/Q or aim for things like "Chance of critical hit" or WSC?
It depends on the WS. For some WS the fTP is transferred through all hits. This then affects multi hit from external sources (i.e. not from the WS itself).
Example:
Blade: Shun transfers TP through all hits. That means if you use Fortia Neck/Waist you have an fTP of 1.2 that applies to all hits, i.e. to the 5 nativ hits + and double/mutli attack proc.
Blade: Hi on the other hand has an fTP that does not transfer (damn if it would that would be so OP...). The WS itself is just one hit with fTP 5. Any multi attack proc adds a normal melee swing (so as if fTP was just 1). Fortia neck/belt increase the fTP of the first hit! so 5 becomes 5.2. but it does not affect any of the additional hits as the fTP does not transfer.
EDIT: Well noticed it was misleading: "adds a normal melee hit" is not quite correct. As far as I know WS stat mods do apply to all further hits, just fTP is 1. So its not a "normal melee" hit in that sense...