Depending on what Madame Pele is up to, spending a night at Volcano House is a good option. If they don't have any crater view rooms then I would keep with your original plan. Also, the restaurant there is good so you can watch the sunset while you eat and see the glow.
If you have allergies or asthma either hiking down to see the lava flow or a helicopter tour may be better than a boat tour. The boats got pretty close and my wife (who has allergies) said she would have died if we had taken the boat tour.
I would probably take the route Mauna Kea -> Waimea -> Hilo -> Volcano -> South Point -> Kona. That way you get to see pretty much the whole island and all of the different ecosystems. Also since you will be spending a lot of time North Kohala you can check out Hawi. The last time we stayed at Mauna Kea we did the drive to Hawi and saw a bunch of whales.
What kind of food are you looking for and budget? Since you are going to be near Mauna Lani Hotel, I would definitely go to the Captian's Table at Canoe House.