I have looked at the special Old Swedish neuter plural ending, and it
wasn't -u, but rather -n. E.g., when Old Icelandic has 'engi' (neuter
plural of 'enginn' - "nobody"), Old Swedish has 'ängin' (Dalska
has 'inggû'); when OIce has 'þessi' (neuter plural of 'sjá' -
"that"), OSwe has 'þässin' (Dalska has 'isû'). It was the nasality
of the Dalska and OSwe endings that was the common denominator, not
the 'u'.
It may be possible that Dalska 'û' was originally an '-ê'/'-î' (< -
in) which become more widely used than just for a few specific
pronouns. Thus, a neuter plural pronoun like 'ingê' (< ingin) may
have caused a neuter plural pronoun like 'nákur' to become 'nákê'.
But I don't know how 'ê/î' became 'û'. Actually, for some words
this 'ê/î' > 'û' never happened. One example is Dalska 'fiuorê' which
comes from an older 'fíogur' (OIce 'fjögur' - 'four'). One would
expect 'fiuorû', but the evolution stopped at '-ê' by some reason.
So, I agree with you, Konrad and Haukur, in that Dalska '-û' is not a
preserved '-u'. Instead, it is a product of the Central Swedish
tendency of adding a final '-n' to neuter plurals (compare with
Modern Swedish 'äpplen' - "apples"; 'ögon' - "eyes" etc - this love
of using 'n' seems dialectally to be limited to only parts of
Svealand, and not in a majority of Sweden)
-----------------
I would like to correct
"So av eð werið frô dyö Ingguov byggde syöðer (?) i Rätsjwaik."
(Suá hæfir þat uerit frá þuí Ingólfr bygþi suþr í Ræykjaruík.)
It should be
"So av eð werið frô dyö Ingguov byggde suði Rätsjwaik."
where I use archaic 'av' rather than modern 'ar' (MIce 'hefir').