Hi Jonah,
Please note that the following is my own personal opinion and not that of the company, but having travelled to Iceland in mid-February I feel I can share my own experiences as the weather will be pretty similar.
You're right, layers are the best way to go - I practically lived in my thermals. I agree that with layers, a waterproof coat and few fleeces you should be fine. I would make sure that you keep warm gloves, hat and a scarf close to hand. If you are taking the optional Northern Lights excursion wrap up - I wore most of the clothes I'd packed, resembled the Michelin Man, and was just about warm.
It may also seem obvious, but a good pair of walking shoes/boots are worth taking. It can get icy at those temperatures and whilst you may find it is quite clear in places like Reykjavik centre if you choose to take the Golden Circle tour, at this time of year, walking up to see Gulfoss and around the geysers it may be icy underfoot.
I hope this helps.
Julia