So, my insights on rewatching so far!
When they're in the car, it's after Jo mentions getting coffee that Al talks about the meds - my theory is that he suddenly remembers giving Noah medicine while Jo was getting coffee, and realises that he gave him the wrong one - so pins it on Jo. (This does involve him thinking incredibly quickly, and I realise it's inconsistent with him initially not remembering if Noah was awake at the airport. But I stand by the theory!)
It's in the garden that Jo realises, with a blinding clarity!, that he's manipulating her (this is after Alex has warned her) - and she starts acting then. When she meekly says to him in the garden that she thinks she can manage the tv interview with his help, she's playing along.
So, when on the tv interview Jo goes off script and starts asking whoever's got him to bring Noah back, I think that she is being calculating - she's trying to set things up so that she can claim she genuinely thought Noah had been kidnapped, and that Al killed him by mistake. At this stage she's in the process of forming a plan to turn him in, I think.