Not quite. FnP has no wording for after. This is what it says
It specifically says when, there is no reference to after. That must be something you mistakenly added.When a model with this special rule suffers an unsaved wound, it can make a special Feel No Pain roll to avoid being wounded (this is not a saving throw).
Force says:
So by your reasoning then in this case, they must occur simultaneously. As you said,when is simultaneous with the unsaved wound. It must also be simultaneous with the inflicting of the unsaved wound, since once the wound is inflicted, the model suffers from it.If a Psyker inflicts one or more unsaved Wounds with a Force weapon, he can immediately choose to activate it by expending a Warp Charge point and taking a Psychic test (see page 67).
It isn't even a case of after, that word doesn't even occur in any of the rules.
That should be more than enough justification for FnP occurring at the same time as Force.
In fact, the argument that FnP is before Force is stronger, simply because the rule for Force has a pause, all the player does is immediately choose to activate it, it does not say you immediately take it. whilst with FnP it is automatic, you cannot choose not to take FnP.
The way I see it, the rules work like this:
Force: inflict unsaved wound -> choose between to activate or not -> activate if yes
FnP: suffer unsaved wound -> roll FnP.
Thus the roll for FnP occurs during the choice of activating Force, prior to it actually being activated, thus by the time you want to activate Force, the wound potentially no longer exists.
Or it can be agreed upon that they do occur at the same time, thus refer to page 9 for simultaneous rules.