As for my idea on the konunginn, it was rather a matter of instinct. I
think Ive never seen such a complicated case related to the usage of
the articles. I have been studying two languages so far which dont
even have articles (Finnish and Estonian), and it seems to me that
accepting the rule that there is no article at all is easier than
understanding that the usage of the articles differs some way
comparing to an already spoken language/mother tongue.
Ive tried to recall the proper rules on the articles in the German (to
make a comparison, and found that there are some very special cases
when the definite article is not used, even though it should be
(because the persons/things are already introduced). For example:
Gewerkschaft und Arbeitgeber haben sich vereinbart.
[(The) trade union and (the) employer have come to an agreement
]
or
Flora und Fauna des Landes zeigt große Vielfalt.
[(The) flora and (the) fauna of the country show great variety].
There are some other cases when the definite article is not used, I
can't recall anymore at the moment. But as soon as Gewerkschaft
(employer) is to be put in to accusative etc. a definite article must be
used. E.g.:
>< Gewerkschaft und >< Arbeitgeber haben sich vereinbart dass es
keine Entlassung geben wird. Die Regierung hat bei den
Verhandlungen >die< Gewerkschaft unterstützt.
[>(The)< trade union and >(the)< employer have agreed that there will
be no job cuts. The government supported >the< trade union at the
talks.]
Briefly, I thought something similar could be in the case
of konung/konunginn.
As to your last sentence [Or is there a really obvious rule that we're
overlooking?], may be the usage of the articles in ON was not as
matured/settled as in Modern English, and therefore it could be that it
was permissible in ON to shift between definite and indefinite form of
nouns without getting the reader/listener confused.
Imre
PS. Ive had a glance at the grammars on this page so far:
http://www.ling.upenn.edu/~kurisuto/germanic/aa_texts.html. I
havent checked them on syntax and these may be already known to
you.