You could be dealing with something called nonallergic rhinitis. Essentially it's nothing more than allergy symptoms that are not caused by allergies. It could be that the air itself from the CPAP is irritating your nasal membranes and sinuses. And in that case, a small bit of humidity might do wonders.
When you were using the humidifier, what setting did you have it on? Did you ever try putting water in the humidifier, but leaving the humidifier set to "OFF"? This is often referred to as "passover mode" and it ads a small amount of moisture to the air, but does not heat the air at all.
Personally, I'd drop all the ineffective meds (in part to establish that they really are ineffective). I'd also start using a saline nasal spray whenever the nose starts to feel congested or whenever you start sneezing. You might also want to add a neti pot or sinus rinse to your daily routine. Either of them would help soothe the irritation and should help with the sneezing.
current settings Min EPAP = 4, Max IPAP = 8 and Rise time = 3
8/1/2010 sleep study results:
AHI = 3.9 [AHI = (#OA +#CA + #H w/desat) per hour]
RDI = 23.4 [RDI = (#OA +CA + #H w/desat + #H w/arousal) per hour]
Dx: Moderate OSA