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1) can a desperate blow charm used on missile attacks or damage tests?
2) can a surprising attacker use the agressive attack option? in most cases that absolutely worth it. (about guarantees an armor defeating hit)
3) can a character "change his action" to delay it until the next round? (oh, things going south fast, better act first next turn instead so i can run away)

1) yes
2) yes, however: that should be announced before the "ambush" is performed, so it can con the wrong way if the targets make their PER Test
3) Yes, see Delayed Actions p. 217. Note this is something different from Changing Actions.

many thanks for the answers! (as usual )

I'm not sure what you mean with "silent walk versus his perception", if that means you roll it against his PER Step or if you mean to make 2 PER Tests, one for Silent Walk and one for Surprise.
in any case: you never roll Silent Walk against something actively (Silent Walk works passively), and simply using it does not prompt a test (only the situation does, in this case Surprise). In essence: Silent Walk replaces any Difficulty Number to hear you with the result, but the target does not get an automatic test to detect you either "just because you used Silent Walk". You'd only make a Test for the guard if he would make one anyway, independent on if Silent Walk was used or not.

As in Surprised, the target is surprised if it does not detect you, you'd simply roll Silent Walk and say you move in, then make a PER Test for the guard to see if he hears you, and that's because the Surprised rules prompt the test. This Test is made against the Difficulty required to hear you at that moment, which is normally your DEX Step. But you have used Silent Walk, so this is not a normal situation, and he rolls against your Silent Walk Test Result, as that has replaced the normal Difficulty.
So, you just use the Surprised rules, filling in "Silent Walk test result" in place of "Dexterity Test".
Note, though, that this can still be modified by conditions from the Perception Difficulty Table in the GM's Guide. For example, in loud surroundings, you could get a bonus to the Silent Walk result.

A) u say : 'So, you just use the Surprised rules, filling in "Silent Walk test result" in place of "Dexterity Test" '. If i read correctly in the rulebook, its not dexterity test but dexterity step, right?

B) so if i sneak upon a guard and dont use silent walk (from now on SW), does the guard roll versus my dex step? and otherwise rolls against the SW test result?

so, lets consider a guard that walks along a wall (around a small building), the wall being parallel with his movement, he is converging to a corner where he will turn.

C) i wait for him at the corner where he is headed, and when he turns the corner I attack him, without using SW. How to determine if he is surprised? Just rolls a perception versus my dex,to see if he is surprised, right?

D) lets suppose the same again, but this time i try to use silent walk somehow. Can I? I dont walk or anything, just wait for him.

E) This time I try to sneak upon him from behind (using silent walk). in this case he rolls a PER vs my silent walk test result to determine if he is surprised and then combat can begin?

A) right, Step, sorry

B) Yes, but note: the DEX Step is the default thing, you can always determine something else, and apply modifiers from the general rules for Perception Tests. Under most circumstances, it'll be DEX, though.

C) Yes.

D) You can't, because the ambush / Surprise situation is not about being heard, it's about hiding behind the corner, it's about spotting you. PER vs. DEX still applies (DEX is also your default for hiding).
Sidenote: Walking is not a requirement for SW, SW can be used to stand still unheard (for example, for a windling, muffle the sound of his wings while hovering), but it's always only about acoustic perception.

E) Yes.

I'd like to add real quick that Silent Walk has a talent knack that gathers shadows around the adept (there have to be some present, or the light can just be really dim). This allows the adept to use the Silent Walk talent to modify visual Perception test modifiers with the same Silent Walk test.