My teacher argue that the vowel triangle for vowels in final syllable was /e/, /o/, /a/ before 1200. He say the oldest text are pretty consequent about using e and o, where later icelandic use i, u.
example köllom > köllum, lande > landi.
He say, that in 600 - 700 while the i-mutation still was productive, old /e/, and old /i/ was still separated. But after the i-mutation became unproductive, they merged into /e/, and old /u/ and /o/ merged into /o/. And then first later, they was raisen to /u/ and /i/.
That do explain why we have words with /i/ without i-mutation: hundi, landi, faðir
However, i have not looked at so many texts, and neither has he. The icelandic homilie book conferms what he says, but what do you think?
I think it is possible tht old /i/ and old /e/ have merged into the modern icelandic /i/ which sounds half like an /e/. About /u/ and /o/ i dont know what to say. We do got examples of words with /u/ but no u-mutation.