> Eftir það ríða þeir á þing upp og hafði svo nær að frændur Þorvalds

mundu ganga að þeim.

> After that they ride up to (the) Thing and (it) would-have (been) so

near that Þorvald's kinsmen would go at (attack) them.

MM & HP: "Thorvald's kinsmen were on the point of attacking Thangbrand,"

> Braut fyrir bjöllu gæti (gætir),
> Broke for (the) guardian (linked to gæta?) of (the) bell,

That's right, and yes - exactly as you suspect - 'gætir' it's an agent
noun derived from 'gæta'. Thor (mög-fellandi mellu = son-slayer of
giantess, slayer of son(s) of giantess) is the subject of 'braut'.

> val strandar,
> hawk of the coast (ie the ship)

Theoretically it could be "hawk", although 'valr' is also a 'heiti'
(poetic term) for "horse", which would make a more normal sort of
kenning for ship.

> mástalls Vísund allan.
> (the) entire Vísund (Bison, CV) of (the) mew-manger (ie the sea)

(altogether bison of sea = the ship)

That's right. 'stallr' can also mean a pedestal, making this a kenning
of the type "land of gull" = "sea". Your translation is clearer and
more literal here than mine here since 'allan' (accusative) agrees
with 'vísund' "bison". I paraphrased it in my English version as
"[broke it] all up".

> Hlífðit (enclitic?) Kristr,
> Christ protected-not,

Yes exactly, the enclitic particle -it, which can be added to verbs in
poetry to negate them.

> þá er kneyfði knörr
> when was-swamped (the) merchant-ship,

"when [he] crushed the merchant ship" (Lex. Poet. knuge, undertrykke,
slaa i kvag) [
http://www.septentrionalia.net/lex/index2.php?book=e&page=341&ext=png
]. Compare the agent noun 'kneyfir'.

> málmfeta (málfeti? ) varra.
> (the) pacer? (CV) of oars (ie ship)

According to Lex. Poet., the variant reading 'mál-feti' is "not found
in the oldest and best manuscripts", although it appears elsewhere in
a list of 'hesta-heiti' "poetic terms for horses" [
http://www.septentrionalia.net/lex/index2.php?book=e&page=391&ext=png
]. Iron-foot apparently refers to a horse shod with iron, although
it's also speculated here that it could refer to an iron fittings on
the prow of a ship.

> Lítt ætla eg að guð gætti
> I little think that God watched-over

Good.

> Gylfa hreins að einu (see Gordon, under einn).
> (the) reindeer of Gylfi (ie the ship) in the same way

You're getting the hang of these kennings. I think this is a little
different from Gordon's example of 'at einu' though. I paraphrased
'lítt ... at einu' "not ... at all", but literally it's "but little",
"only a little", see Lex. Poet. 'einn' 12 [
http://www.septentrionalia.net/lex/index2.php?book=e&page=103&ext=png
] and Zoega 'einn' 7 (end of entry: 'at einu' = 'at eins' "but, only").

"But little think I that God guarded your ship."
"I think God watched over your ship only a little."

An understated way of saying that he didn't look after it at all!