Interesting. The idea of neuter singular as the least marked
combination of gender and number ties in with its use for past
participles with the auxiliary 'hafa' to form the perfect tense, and
in the passive with verbs that select dative or genitive.

(1) mönnum varð borgit flestum
"most of the people were saved"

In Gothic too there are a few instances of natural gender or number
winning out over grammatically "correct" forms, as well as scattered
examples of neuter past participles used as a default where another
gender would be expected.

(2) þannu gatauran ist marzeins galgins
"then the offence of the cross is destroyed"

(3) ei kanniþ wesi...handugei gudis
"that the wisdom of God be made known"

(4) at þaimei gatarniþ ist sunja
"from whom the truth is lacking"

(5) iþ ufarassiþ <warþ> ansts fraujins
"and the grace of the Lord was abundant"

In each of these examples, the Gothic combination of auxiliary verb
and past participle corresponds to a single verb in the Greek
original, although normally when this happens grammatical gender
agreement is observed. In (3), the past participle is separated from
the (feminine) noun by quite a long way, which might have something to
do with it.

Regarding number, Richard Nielsen posted to Old Norse Net a while back
a list of examples of singular verbs used for plural in Swedish
diplomas of the 14th century [
http://lists.mun.ca/cgi-bin/wa?A2=ind0710D&L=ONN&P=R71980&I=-3 ].
Unfortunately the context is missing for some of them, but a lot of
those that do have some context are in relative clauses, in particular
a certain kind of legal formula. I don't know if that's an artefect of
the search method or the nature of the corpus, or a genuine tendency.

(6) Them sum epter komber
"those who come after"

(7) þøm þær æptir comber sum þøm þær nw aer.
"those who come after even as those who now are [living]"

If it is significant that number-agreement occasionally fails in such
contexts in Old Swedish, and not just the first signs of the general
decline of the plural in spoken Swedish, I wonder if it might (at
least sometimes) have something to do with confusion between the
concepts of "each" and "all". "Each" is singular, "all" is plural, but
they often amount to the same thing:

(8) each of those who come(s) after
(9) all of those who come after

Another factor in the case of (8) is the ambiguity over whether the
subject is "each" or "those". Another English example that comes to mind:

(10) {there's, there are } loads/lots of them
(11) { there's, there are } a lot of them

In (10) the first option is just the regular colloquial use of
singular as a default. In (11), the prescriptively erroneous plural
could be motivated by the empatically plural sense of the
grammatically singular noun, but might also be a hypercorrection
formed by analogy with (10).