Magnitude

[T Prime]

so you NEVER add the magnitude of the spell?

i.e. if the target is willing?

The powers listed in the book do not have any instances where Magnitude gets added to the saves made by friendly models.  For the most part friendly models  don't make saves at all.  Powers that could confer a magnitude bonus were not included in this volume as I recall.  Additionally there are no powers that have the magnitude itself adding to the PW check.

[T Prime]

I have another sub question:
On page 86-87 (Save vs. Channel) is written:
"The model targeted by the Power would attempt to roll less than or equal to the listed Stat to resist the effects of the Channeled Power, remembering to subtract the Magnitude from the roll."

The magnitude is subtracted from all unwilling save test vs. the Power (LD, PW, AR, other) or only from test against LD + PW. Or just from LD? I did not notice anywhere, that the Magnitude is not substract from AR (in save tests), but it is logical (when some variable powers increase their DMG vs. AR only aftes adding magnitude - and not in the same proportion).

Magnitude is always subtracted from the channeling models PW to determine what is needed to succeed.  Effects that are directed at enemy models and cause them to make a save have that save reduced by the same magnitude against the relevant stat.  Two examples-

Confusion Power;  Save vs. LD.  Models targeted by this spell must roll their LD (-5 from Magnitude) to see if they succumb.
Etheric Lance; Save vs. AR.  Models targeted by this spell must make an AR roll (-3 from Magnitude) to see if they succumb.
Powers that have environmental saves are not modified by Magnitude.  We'll clean these up in the revision, sorry for the confusion.

What for is the PW stat in non-channeling models? Only for info? I didn't see any power with PW save test what it suitable against normal models (only against channelers or supernaturals).

It is a function of their profile, all models have base stats in this categories.  Some powers require saves made against PW.  When this causes the save to be 0 or negative, a 1 will be required to resist.  In UWZ I know Stain of the Unbeliever comes to mind as one power that targets PW ( and targets it hard).