Let the grammatical weirdness commence ;-)

> því að þeim mega það mest málaspell verða (syntax unclear,

particlarly mega, is it 3rd pers plural or infin?).

> because for them that will be able to become the greatest

flaws-in-the-suit.

3rd person plural, curiously enough.

"because that may become the biggest flaw in the suit for them"
"because that may prove the biggest flaw in their suit"

By a quirk of Old Norse grammar, 'mega' here agrees with 'málaspell'
(neuter plural, copular complement of the infinitive 'verða'), rather
than with the singular subject 'það'. Compare 'þat eru bylgjur' "those
are waves"; 'þat eru hrímþursar' "those are frost giants" (lit. "that
are waves" and "that are frost giants"); 'fólk þat er Ljósálfar heita'
"that people who are called Light Elves".

> "Far þú svo með víst," segir Skarphéðinn.
> 'Manage (you) (it) thus, certainly (fara með, Z18?),' says Skarphéðin.

Yes, I believe so.

"f. með e-u, to do (so and so) with a thing, to deal with, manage"

> "Erfitt mun þeim veita að ganga í mót giftu þinni,"
> '(It) will prove (see veita, Z8) difficult for them to go against

your luck,'

Right.

e-t veitir e-m þungt, erfitt, it proves hard, difficult for one (Dönum
veitti þungt atsóknin); impers., keisaranum veitti þungt, the emperor
had the luck against him; e-t veitir erfitt, it is hard work;
Geirmundi veitti betr, G. got the better of it, carried the day;