I live in Harbin and so have lived through a few Ice festivals now, including the current one. There are three main places that are worth seeing. One is Ice and Snow World (which is the one that most often pops up in the media). The others are the Snow Scuputres on Sun Island and a smaller Ice Lantern display in Zhaolin Park. If you were really disiplined, you could see them all in two days, but I'm not sure I'd recommend that. Three or four days at least would be better. Lots of businesses and govenrment departments/parks, etc will also commission pieces out the front of their buildings.
The obvious place to stay would be around Zhongyang Dajie/Street. It's a nice area with plenty of shops and restaurants along with a fair few restored colonial Russian buildings. It's also near Zhaolin Park and St Sophia Church, which is worth seeing (it's not an active church now, but it has lots of photos showing Harbin from when the Russians came to WWII).
Sun Island is only open during daytime, while Ice and Snow World is open day and night, so you could do them both at once since they are quite close to each other. However, they are both on islands in the middle of the river and can get quite windy and extremely cold. There's also not much in the way of shelter to escape from the elements (besides overpriced restaurants). Sun Island is a fun park/camping ground in summer while the island for Ice and Snow World has next to nothing once the festival is over. So I'd probably do them on seperate days (they're pretty big too).
Looks like you've booked your tickets, but just to warn ye that the crowds get bigger during the Chinese New Year golden week (4-10 Feb 2019). Also, if the weather gets too warm (i.e. near 0 C) and affects the ice, the parks will get shut down. Last year that was at the very end of February and this year it might even go into March (quite cold this year!).