Allegedly (see https://forum.duolingo.com/comment/13265760?comment_id=39113797) the Swedish rot and English root each come separately from words with w at the beginning like Wurzel. Sounds like rather a coincidence to me.
'moronen'. yes, 'au' is a common way of creating a plural in Welsh, but not the only way. sometimes the singular version is longer than the plural, e.g. moronen/moron. another example is mochyn = pig, moch = pigs. Some plurals are completely random and must just be learnt, e.g. ci = dog, cwn = dogs