Sorry to hear of your fall, it sounds excruciatingly painful.
Codeine is an opiate and can cause nausea and also constipation but its a very effective pain killer and needs to be taken regularly as prescribed for the best results, your GP can also prescribe you a laxative and Cyclizine for your nausea it works very quickly and if taken once a day you will feel no nausea again while taking the codeine.
As for sitting either a neck pillow or you could also try placing a cushion either side of your chair, one for each cheek so to speak, so that your coccyx is not touching the chair base. Or how about laying on your side on a sofa, that could work perhaps.
Hope it becomes more comfortable for you soon