[fifiste]

Yes And I'm all fine by that. It's a sound reasoning. What I'm more worried about is that +8 from TA and AoA:D, will still be too small to counteract -5 untrained, -5 from skull hit location. (worse if you add some improvised weapon penalties)
It's still going to be only 26% chance to hit for something that is rather basic movement (take pan with two hands - apply to the noggin right before you)
26% is still chance that makes sense to try and after trying it 3-4 times it will have either succeeded or distracted the black hat enough to give white hat his chance - but what I think would be more appropriate is to give a chance for evaluate or other bonuses. To get a hit chance above the 50% that seems much more suitable for such a simple action.
Evaluate would work fine as in this case the untrained will trade the disadvantage of waiting extra time for better guaranteed hit - which will not break my suspension of disbelief.
The untrained one would still be disadvantaged enough - having to wait several seconds, foregoing his/her own defenses and doling out an attack that would be easy to defend against if the victim would be able.
It would nicely separate the untrained one from a professional who could dish out a similar hit in a second and not leave him/herself defenseless - without making a rather basic action unbelievably unlikely!
So what my point is for situations like that there should be possibility to get more that +8 from TA and AoA:D, either letting evaluate stack with TA or some other bonus. Letting evaluate stack with TA would have nice symmetry of exchanging 3-4 seconds of 1/4 chance attacks to one more guaranteed attack - and would keep the untrained one disadvantaged by turns wasted compared to trained one (so no freebies), but still able to perform tasks that should be performable by anyone with basic hand-to-eye co-ordination.