> 1- efa yður: wonder why, be surprised, or simply "doubt" ?

Simply "doubt" I think. He wants them to trust him, so he insists that he will put his trust in Kári, and tells them not to doubt that.

> eg á fyrir sjálfan mig að svara

Magnus Magnusson and Hermann Pálsson paraphrase this "for my own safety is involved too".

> en kvað sér vel hug um segja ef slíkir væru margir.

"but said that he felt hopeful if there were many [others] like him"
"but said he felt that it boded well if there were many like him"
"but said he had a hopeful feeling if there were many such [as he was]"
(See Cleasby/Vigfússon 'hugr', IV.)

> Riðu þeir þá í braut og fyrir norðan jökul og svo uns þeir komu þeir komu til Svínafells.

"They rode away then and north of the glacier and [continued riding] thus till they came to Svínafell."

'uns' is a modernized spelling of Old Icelandic 'unz' "until, till".