Checking further, I find that in MnI (and presumably ON), the 1pl pres. act
of <hefja> yields <hefjum>, not <hófum> which is past tense, but that's
still not <höfum> so I think your point still stands.

Alan

> Ok svá á mínar systr it sama, en þegar it fyrsta kveld er þér komut
> hér þekkti ek Sigrgarð, ok höfum við mjök ójafnt.

> And also on my sisters (plural) the same (ie likewise), but at-once on
> the first evening when you (plural) came here, I recognised Sigrgarðr,
> and we begin (hefja, Z3) very unequally (guessing: this time round, we
> start on a completely different footing?).

I was wondering what you’d make of this, but I don’t think that <hefja>
works: it should yield <hófum>, not <höfum>.
The latter is, on the other hand, the 1pl pres. act. of <hafa>. (Not that
I’d be shocked to find a non-standard usage in this saga, mind you!)