I do this route on a regular basis, my last flight NZ80 due to depart at 8.30pm and the first that day for Air NZ, checkin did not open till 5pm, other times on a 6.30 flight the earliest counter has been 3.30pm, they are always in aisle G, so I think it would be quicker and easier to take the train and check your bags in at Kowloon, its not that long, you can not check them in at Ysing Yi

As far as the lounges in HK, as a Gold member I have tried them all, I personally like the United lounge