it doesn't matter at all if his legs touch the back of the seat, or if he has to bend them. afaik no child has ever broken a leg in an accident due to being rear-facing, and if they did it's better than the possible alternatives!
he hasn't outgrown his seat until his head is over the top of the back- even then it is dangerous to put him forward facing until he is 9kg/9months
My son is 10kg at seven months and bursting out of his first seat but we were advised by the fitter at Mothercare that he shouldn't go to the next seat until he is able to sit unaided for 5 - 10 minutes. So he is still getting crammed into the old one!
9kg is the minimum - most first stage seats will be fine until 13kg (but most children outgrow in length before weight) - when calculating length you only go on now far his head is from the top though, the legs are irrelevant.
DS fitted into his seat until he was 18 months, he is small, but he's always been on the 75th centile for height. The seats themselves say they are suitable up to 12-15 months.
it';s about them being strong enough to bear the force of a crash.
if you have a head-on collision your babies head will be flung violently forward. this can cause internal decapitation which i believe is the biggest cause of child deaths in car accidents (i read that someone but not sure where so i might be wrong)
it really, really, really isn't worth the risk. i think we're the only country that advises forward facing so early. in america they have to be at least a year, and in scandinavia and thereabouts children normally travel rear-facing until they're about 4
Yes - I started leaving the seat in the car after a while you'd have to do that with the next stage seat anyway! And it's useful to have the option to bring the entire seat in if they're asleep rather than having to sit in the car with them for half an hour.
In an accident, I'd be far more worried about DD's head and neck than her legs. Even though her head control is fantastic, if you're in the sort of accident that can give an adult whiplash, it doesn't bear thinking about what would happen to a baby.