> Honum liggur eftir hnífurinn og beltið.

This illustrates a topic that came up lately: a singular verb with
more than one subject, namely 'hnífurinn og beltið' (older forms:
knífrinn ok beltit).

honum liggr eptir = liggr eptir honum (dative). I think this might be
what the jargon calls a "discontinuous Prepositional Phrase"; compare
the example I quoted recently from Faarlund: Old Norse Syntax, p. 148:

þá mun ek eptir leita þeim eignum og öðulum, er [...]
"then I will seek after the property and inheritance which [...]"

For more examples, see Eiríkur Rögnvaldsson 1995: `Old Icelandic: A
non-configuratiuonal language', NOWELE 26:3-29 [
http://www.hi.is/%7Eeirikur/ ], e.g.

var mikit til aflat þessar veizlu `much had been provided for this
feast' (Laxdoela saga 27)

...UNLESS perhaps the example from Njáls saga could be interpreted as
adverbial 'eptir' with the dative of respect / possessive dative, as
in: 'Hann vaknar ok spyrr hvárt laufsblað felli í höfuð honum' "He
wakes up and asks if a leaf fell on his head"?

LN