Regarding the mutual intelligibility of Old Norse and Old English, Guy has it right. The answer has to be that it is complicated, if not fruitless, to try to work out whether they were really mutually comprehensible, and dialects make things even more difficult.
Just because it’s fun, here’s a 15th-century misunderstanding between a northerner and southerner in England. The northerner’s dialect includes Old Norse words that make mutual intelligibility hard: http://www.bl.uk/learning/timeline/item126611.html
Paskal, yes, I was stating that language was one of the main markers of a given ‘ethnicity’.
On the fashion front, we don’t know much about ideas about fashion, although Ælfric’s letter to Edward could suggest that people were just as eager to follow the latest fad back then as they do now.