I'd want to know on what grounds the continence specialist determined that it was 'behavioural'. I dunno, it could be. But it sounds odd to me and without anything that's obviously triggering or reinforcing a behaviour (stress, need for control, attention issues...) then like you, I'd be wanting a really thorough assessment of bladder, control and frequency issues....
I know of a few kids who have regressed because they went too much 'just in case' and therefore the assessment was that they lost the physical control and awareness around completely full, vs completely empty. If this is the case, then toilet timing is going to make it worse. If it is behavioural, then taking her regularly might be the way forward. I feel like calling up that nurse for you and demanding a better explanation for her fobbing you off. Grrrr.

Did you end up finding an OT for DS? An OT also may be able to assess physical vs behavioural issues of TT, and could help you make a plan to at least test hypotheses.