> Back to Old Norse, there's a curious example in Skáldskaparmál where
> neuter singular used in place of masculine plural. I don't know why
> that is. It doesn't seem to be a conflict between natural gender and
> grammatical gender.
>
> #10 hér er kallat hvalir Viðblinda geltir
> here whales are called Viðblindi's boars
> (Skáldskaparmál 46)
>
> I wonder if it's a similar phenomenon to the English fluctuation
> between singular and plural in sentences like the following, #8 being
> a colloquial alternative.
>
> #8 there's four of them
> #9 there are four of them

Yes, I think you could say it's a similar phenomenon. I think in both
cases we're dealing with 'markedness'. In English (and other languages)
singular is the unmarked number so in some dialects and some contexts it
can be used where plural would technically be called for. I think in Old
Norse the neuter singular of adjectives might similarly be an unmarked
default form.

Here are some more examples from Snorri's Edda:

"þat var í forneskju kvinnabúnaðr er kallat var steinasörvi"
"ljóð þau er kallat er Gróttasöngr"
"Af hennar heiti er kallat snotr kona eða karlmaðr sá er hóflátr er"

(Grabbed from a Google search, I haven't checked this down to the
manuscripts or anything like that.)

Compare with this sentence from Sæmundr's Edda:

"Helgi ok Sigrún er kallat at væri endrborin"

This is slightly different, isn't it? We can parse it like this:

"Kallat er at [Helgi ok Sigrún væri endrborin]"

Yet it seems like a short step from this easy-to-parse example to your
hard-to-parse example.

Kveðja,
Haukur