I've been up Haleakala twice at 10,000+ feet and what impressed me was the wind, so I would prepare for strong wind and wind chill lowering the effective temperature.
Someone mentioned a poncho and, as much as I like them, I wouldn't bother in windy exposed terrain like that. IMHO, the best "what if" rain option is a DriDucks jacket. It will block wind too.
The wool base layer isn't bad, but you might consider a polyester one that would be better to use at home-- a silkweight like Capilene 1. Then layer that with a mid-layer like R-1 or Power Stretch, or wool. At 20F I would want a puffy layer-- your softshell will provide some wind and water resistance, but it won't provide much insulation. In fact, I would have a windshirt and a puffy layer rather than the softshell. How much puffy depends on your other layers and your comfort. If I had a base layer plus a mid, something like a Micro Puff or down vest would be good, YMMV. The windshirt should have enough room for the layers and avoid compressing the puffy layer. Softshells aren't as good for layering with lofted insulation unless they are light and have lots of room. You could wear a vest on top, but it will work better under an outer shell.
If you aren't used to cold, good gloves, beanie cap, or a buff or balaclava will make things cozy. An extra pair of thick socks will help for camp and sleep. When your extremities are cold, it makes the perception of cold worse I think.
If sharp volcanic rock is a concern, a CCF foam pad might be a better bet, or one in combination with your air pad.
The cabins may have foam mattresses (http://www.travelblog.org/Photos/3576176) and you will be protected from the wind much better than any tent. At any rate you'll be sleeping on wooden bunks and not on the ground. I would be okay in a 20F bag, but if you aren't used to it, a warmer bag makes sense to me-- depends on the bag. Perhaps you can rent one--- it seems a shame to lay out big bucks for one trip and a couple nights.
Bob's advice on the caves is a gem if you are not able to make it to shelter. Blown dust and pumice may make sleeping outside a shelter a pain and watch your camera too-- volcanic dust can jam the lens. I've woken up with windblown sand in my ears and everywhere-- nasty.
Have fun, and catch the sunrise--- awesome in the true meaning of the word.