with only one dc in, you can take one seat out, put the other seat on centre track, so all more stable, iyswim?
erm , number of bags wise do you mean weight-wise or space-wise? 
obv weight wise depends on size of dc etc.
space wise, well I used to have: both seats in , one seat pushed quite far back as dd1 is tall, other seat adjusted to about middle I suppose. could get 2x poncey wicker/woven baskets in the back, or about 4/5 supermarket bags easily. you could pile more up height wise, quite easily if you were so inclined.
seatbelts in mine are 3-point - around the waist and crotch strap. I notice now on the site they have a 4-seater bike, which has 5-point seatbelts, don't know if available on the 2-seater - it doesn't look like it.
but I have never had a problem with the 3 point belts - dd2 was only 2 when we were riding lots, and she sat snugly. dd1 was also fine, she was about 4 I guess, but with quite delayed understanding due to SN.
kerbs: not great. you need a proper entry point. I have tipped mine once (coming downhill, quite fast, misjudged angle onto footpath/cycle path, camber of road was nasty and bike juddered up over the kerb, and tipped. I was going too fast though, ans could have avoided it otherwise)
tracks - my old usual route was 1/3 cycle path, 1/3 road, 1/3 route across common/through woods. it's ok on an ok path/track. won't do deep ruts well, very heavy in mud etc. but for eg a decent forest path it was fine - a bit juddery, but no more so that a cycle trailer I shouldn't htink (haven't ever used one, so can't really compare more than that)
cost wise: about £2k a few years back. very expensive, but I hardly used my car all week when we got it, as it was everythign I needed. so when you price it up against a small car (if possible) then it is a good price.