I think the rain is very regional and it comes in intervals. I came back from HK less than a week ago (I took the kids there for a week), and when it was raining, we had to hide somewhere. It took like 1 hour or something then it was dry and even sunny (people were commenting on my little girl's sun tan).

My advice is to get closer to where you want to go - e.g. if you want to see the peak, head to admiralty, then check the weather when you are there, if it looks good, continue up to the peak. If not, stay at the surrounding area a bit, wait until the rain is over, then head to the peak.

It doesn't get really dark in HK until something like 7:30pm, so you'll have enough times/attempts to try.

If the weather is too bad, you can go to the lobby cafe at Intercontinental HK, where you can buy a drink and enjoy the view. I always find the view from Kowloon to HK nicer than vice versa.