I would definitely persevere getting her onto wet food if you can, dry food is dehydrating if fed exclusively. Persians can be prone to tummy probs as they are particularly sensitive to the carbohydrates in many cat foods (especially dry). I found changing over to a mainly wet diet (raw mostly with some pouch foods) made a hec of a difference to my persians.
Some of the higher meat wet diets might suit Pixie, maybe Hi Life real pouches, Natures Menu etc, anything really that doesnt contain carbs (cereals/grains etc). Unfortunately all the dry foods contain carbs, although Applaws dry and Orijen dry have lower carb content, but, as persians can also be prone to kidney problems, I'd not make dry the main part of the diet. Mine have a few biscuits as treats only.
Could you trim the fur round her back end for the time being just to avoid her getting too messy there? either use scissors (carefully!) or this little battery trimmer is quite useful (for the odd small knot or two too)
http://www.amazon.co.uk/Wahl-Trimmer-Battery-Rubberised-8066-717/dp/B000HC2MDE