Yes a "mere common coachman" not James merely means that Mr W regarded all other coachmen as inferior to his own servant..
As for carriages, as a bachelor Mr Knightley can do without using his carriage, he can ride around and walk. But when he has a lady in his family he will need to make use of his carriage.
John K is a sensible saving man who wont live up to his income, but will try and make savings to provide dowries etc for his daugthers and money for his younger children, in the future, so he's not willing to go further than he has to, to give them a holiday. He's not as rich as Mr K, naturally but IMO Isabella has the same money as Emma.. Mr K will have Emma's money, any extra that Mr Woodhouse has, and hsi own large estate to keep him in comfort, but he too isnt extravagant and will save for the future and put money inot the estate.