stroðinn and sorðinn are perfect illustrations of the process I have
identified as occasioning the presence of the thematic vowel; namely, a shift
right from C'VC(V) to CV'CV signifies active versus passive involvement
with the action.
If we start with a theoretical *soroð, stress-accenting the first syllable
produces *sór(o)ð, 'coupled (one)'; stress-accenting the second syllable
produces *s(o)róð, 'coupler', into which a -t- is inserted for ease of
pronunciation: stróðinn.
Patrick