...the moment you don't quite trust your indications you should trigger the procedure application.
Agreed. But what actually defines “the moment"? I think the OP was looking for definite trends/indications/events that might alert experienced pilots to a developing UA scenario.
I was trying to make the point in my previous post that we experience environmentally-driven UA-like symptoms far more often than we do actual UA itself, therefore naturally have trouble diagnosing real UA when it happens as we are exposed to “fake” UA on almost every sector.
One of the points of this debate is to ask why 'the book' has been changed.
Possibly because of incident/accident trends and a de-skilling of the ‘traditional’ way of flying? Or that it was recognised that in a high-stress scenario, having something simple and widely applicable might lead to a better overall performance from an average crew?
In older aircraft, you just took the AP out (if it was in in the first place) and carried on flying, pretty much. With FBW, autothrottles, envelope protection, etc. that may not be enough as the protection systems could be trying to kill you and you have to positively assert control over them.
Regarding TCAS vs. UA drills, I think the other aircraft involved will get their RAs modified in the light of you not following yours. Also, as TCAS uses pressure altitude, any static problems will render it fairly useless.