I have the nordic cab version:
>Are they big enough for two rather big children (say three year olds)?
We have a 5 year old and a three year old they now squeeze together!! when they were younger it was a perfect fit, only problem is now the kids elbows sometimes push the outside to the wheel
>Can you fit a 6 month old baby inside comfortably?
Yes but you need the soft carry seat I have one if your interested ?
>Is suspension really necessary (i know NC doesn't have it, but apparently suspension >kit will be released this May)? Can just a very fat tire do a good job with two one >year olds inside + luggage?
Tyres are fine no need for suspension the NC has a soft padded seat system inside anyhow the ids love it!
>Is the hard trailer's bottom a big advantage?
Makes it more like a rollercoaster ride for the kids
and keeps there feet safe
>How stable is it (especially the Nordic Cab)?
Very I haven't managed to roll it yet!
as the weight is on the axle the low position makes for a fairly stable bit of kit
>How appropriate is it for a wet/snowy winter conditions?
Snow not sure but wet no problems at all there is weather proof flaps all around
>Is it really good for jogging, do you hit the back wall of the trailer when running?
I dont jog but its nice to push around town the handles are in a better position than and I guess as long as your stride is not to big!! its also more moveable because of the front wheel
>Is it appropriate for long distance self supported touring?
Without the children would be ideal for dumping your camping gear with children yo would be limited in storage space I guess count on a small shopping trips worth of space with two kids