Is there a way to separately query those for which shipping suspension was requested in the product preparation stage and those for which return was requested after shipping ?

When the product is getting prepared, shipping is not ordered yet(waybill number registration) thus you can check it in both of the RU(Request to suspend shipping) and UC(return registration). 

When a customer returns after shipping, delivery order was already given for the product and it is not possible to complete shipping thus you can query only in UC(return registration)

Therefore, you can compare 'before shipping(preparing product) and 'after shipping(after delivery order) based on RU(request to suspend shipping) query result.