But here's my gripe about this Keela Ventile with a waterproof/breathable liner: on warmer days or when I am working hard while wearing the jacket, the breathable liner works well at taking away the moisture vapour I produce. All well and good. However, when the vapour meets the Ventile outer, it condenses, prompting the cotton fibres to swell and impeding the progress of the vapour to the outside world.
Similarly, when it is raining, as it has done with monotonous regularity of late, the Ventile fibres swell, again impeding the flow of moisture vapour to the outside. In both situations, the 'breathability' of the smock is compromised markedly.
In both situations, the now-stiffened garment becomes uncomfortable to wear because of its stiffness and because it becomes a little like wearing a plastic bag.
My experience of this particular garment has shown me that, while it might suit a largely static wearer, the combination of Ventile + a waterproof/breathable liner simply does not work effectively and efficiently. Maybe a polycotton outer with a DWR finish would work better; it would certainly dry out more quickly!
I think you nailed the problems. Any 2-layer jacket, no matter of how 'breathable' the waterproof fabric is, will be too hot for anything but mostly static conditions. Add rain on the outside, and you are soon swimming in your own sweat.
A single layer Ventile soon gets a bit uncomfortable, once it 'wets' and becomes stiff and cold-feeling. A two-layer one is more waterproof, but it's also less breathable and too hot for most occasions on the move.
Nothing beats something like the goretex-like laminates on that front; the only reason you might want Ventile is that it's not plastic, it won't end up with very expensive burn-holes next to a fire, it doesn't delaminate, it repairs pretty easily... and the list goes on

Personally, I am a big fan of Ventile, but I have settled on having two identical smocks in single-layer. They are a lot easier to dry, you can layer with one or both, you can have them in two different shades and layer on the outside what's most appropriate, and if one gets damaged or looks pretty tattered after lots of use it doesn't matter too much as you can wear it on the inside when you need both.
On the inside, I use layers of merino baselayer, wool shirt, loden smock. I will usually layer these as required when on the move (on the move, I'll usually actually be just on the baselayer or at most adding the shirt for most conditions), only putting the ventile on if it's breezy and cold or it's raining cats and dogs.
The two-layer smocks -Ventile or otherwise- are probably too much for anything but static or very heavy winter.