My argument was that because she has not had to actually do anything to actively counter incoming attacks, (her ornaments always did it for her before) she will be very unskilled in it.

This is how I envisioned the fight:

1.Moridin comes to the scene, maybe makes some comment about the DO or otherwise shows himself to be a forsaken.
2.Cadsuane realises he is a male channeler and provokes an attack somehow.
3.Moridin throws a TP fireball at her (okay he would probably use something nastier but this is an example)
4.Cadsuane will think the fireball will be dispersed by her ornaments and rather than countering it, will go on the offensive and focus on attacking Moridin instead which is probably what she did when facing those male channelers and false dragons.
5.To her shock and surprise, the fireball will not be dispersed since its not made with OP and Cadsuane will be blasted.

Even though she might realise that no OP was used, that would take time since she has to actually realise that her other channeling detection ornament did not move. Time she does not have with a fireball heading her way.

But even if she did realise it (Moridin might attack someone else first, giving her the time to notice her detection ter'angreals lack of movement for example), after centuries of no need to do anything to block those fireballs , she must be horribly out-of-practise in the process don't you think? Without her ter'angreal and using just her skill, I doubt she could block an attack made even by Nynaeve or any other AS. What reason would she have had to practise such weaving techniques afterall? She does not know that TP exists and her ornaments make her immune to both sides of the OP.

This was my point. That by relying so much on artificial devices to aid her, she has probably allowed her own natural skills in those areas to atrophy to almost nothing.

As for why I think her ter'angreal won't offer protection against TP, I obviously cannot know for sure. However we know from BS that devices designed to counter OP (the FM guardian) wont automatically counter TP. It seems highly improbable to me that whoever designed the ter'angreal would specifically have made them to shield against the TP too considering how rare TP was. Heck, I would be surprised if the designer even knew the TP existed.