I would be in favour of a deep analysis on phonology, especially regarding vowels. It seems a bit un-practical to me to shrink the vast richness of the Scandinavian languages (both mainland and insular languages), considering what has been done until now. Especially concerning what it has been done with vowels.
We must admit that, Jakobsen - at the time he was writing his ethymological dictionary - was not properly in favour of an ethymologically based written laguage.
I have, for this reason, to recall you the first stages of the history of the written Faroese language, which was, at the time, still covered in fierce debate upon which kind of orthography to adopt. There were Jakobsen's and Hammershaimb's proposals and we all know that, in the end, for ethymological sake, luckily the latter was adopted for Faroese.
Jakobsen - I think - probably recorded Norn material according to his own orthography, the one that he used to write Faroese, which was not based on the ethymology of the words.
We now have Aa Ee Ii Oo Uu Yy Øø Åå, without distinction about vowels lenght and/or value, where Old Norse (and, consequently, Old Norwegian and Old Danish/Swedish) had Aa (long: Áá), Ee (long: Éé), Ii (long: Íí), Oo (long: Óó), Uu (long: Úú), Yy (long: Ýý), (long: Ææ - no short), Øø (long: Œœ / Ǿǿ), Ǫǫ (no long).
With nine basic vowel sounds (which, again, can be short or long, nasal or oral), it seems quite strange to me that this has been reduced to that proposed "alphabet".
Yet, I know, many times Hnolt said everything will be re-considered before editing the definitive version of this incredible work.
Going back to my footsteps: it is not so obvious that, ethymologically speaking, all the sounds might be considered by "stepping back" on vowels.
Oo in Norwegian, Swedish and Danish, for example, is uː and not as in Icelandic / Faroese.
Yy on the other hand is not Uu, but yː or ʏ.
And so on...
So, after having done a little bit of researches in this couple of months, this is what I got - regarding Norn phonology of vowels.
Sounds files are taken from Wikipedia, voice is not mine
(x: denotes a long sound)
i: (Close front unrounded vowel) - Ii
ɪ (Near-close near-front unrounded vowel) - Ii
eː (Close-mid front unrounded vowel) - Ee
e (Close-mid front unrounded vowel) - Ee
ɛː (Open-mid front unrounded vowel) - Ææ
ɛ (Mid front unrounded vowel) - Ææ
ɑː (Open back unrounded vowel) - Aa
a (Open central unrounded vowel) - Aa
oː (Close-mid back rounded vowel) - Åå
ɔ (Open-mid back rounded vowel) - Oo
uː (Close back rounded vowel) - Oo
ʊ (Near-close near-back vowel) - Oo
ʉː (Close central rounded vowel) - Uu
yː (Close front rounded vowel) - Yy
ʏ (Near-close near-front rounded vowel) - Yy
øː (Close-mid front rounded vowel) - Øø
œ (Open-mid front rounded vowel) - Øø
"Shortened" sounds almost only occur preceding a group of consonants, just as it happens in Faroese:
Monopht. --- Long ----------- Short
i / y --------> hin [i:] --------> hitt [ɪ]
e -----------> frekur [eː] ----> frekt [ɛ]
ø -----------> høgur [øː] ---> høgt [œ]
u -----------> gulur [uː] ------> gult [ʊ]
o -----------> tola [oː] --------> toldi [ɔ]