What would happen if she slept 2.5 or 3 hours during the day? Will she last that long? With my DD, I would wake her up ever 3 hours to eat. If she took 25 minutes to eat and 5 min for a diaper change, she'd be back down and sleep for 2.5 hours.
Does she fall asleep during the feed (my DD did this and it was a bugger to keep her awake). If so, there are some things you can do, like blowing on her face/ear, jiggling arms/legs/hands/feet, burping her frequently, fake diaper change (remove clothes; undo diaper; redo diaper and dress). Sometimes holding them in a sitting position will rouse them.
At night, are you swaddling, do you have white noise, pacifier, lullaby, a wind-down routine? Is her room really dark? At night, I suppose it is...My DD really responds to the routine (took about 2 days to implement) and if she's really fussy, I'll undo her swaddle walk around a bit and then start the routine all over. Seems to help her figure out that she needs to go back to sleep. And, let her sleep as long as she can at night. What do you do when you feed her at night? We used to do diaper changes too, but it was too much activity for our DD. So, we started using the diaper doublers and now she can go all night. All we have to do is feed her and pop her back in her crib.
Try to nap a bit during the day (easier said than done, I know) and know that Jamie will sort the days and nights out.