> en veður hina báða þá skuluð þér taka við trú."
> and walks through both the others then you shall (must?) take with

the Faith (accept).

'hina báða' is accusative, often used for ground covered or an area
passed through.

Þeir riðu suður heiði þá er Tvídægra heitir.
"They rode south over the heath called Two-days."
(Grettis saga).

'veður' is 3rd sg. ind. of 'vaða', cognate with the English verb 'to
wade'. In ON it can be used of fire, smoke and steam, as well as liquids.

ok óð eldana sem reyk
"and waded/strode through fire as if it was smoke"
(Hervarar saga ok Heiðreks konungs).

Svá er frá þeim sagt, at þeir óðu elda
"It is said of them [berserks] that they waded/walked through fire"
(Hrólfs saga Gautreksson).

> þá játuðu miklu fleiri.
> then many more agreed

That's right. 'fleiri' "more" is the subject of 'játuðu', and 'miklu'
is neuter dat. sg. acting here adverbially: "by a lot, by far", i.e.
"many more".

> innar
> further into the room

Yes, exactly.