Is the viscous, clear fluid with a faint odor, found upon waking in the middle of the night, considered seminal discharge (احتلام)? And are the blanket and bedding affected in their purity if they touched the wet area, despite their apparent dryness?

The apparent substance that came out is semen, which necessitates ghusl (definition) (ritual bath). If you are in doubt about it, you may choose to consider it either semen or madhiy (pre-seminal fluid). The mattress and cover are ruled to be pure, as semen is pure, and it is not certain that madhiy has transferred to the mattress. Impurity is not ruled to have transferred merely by doubt.