I didn't expect to get a reply on this one day
I'm quite a bit further in my learning of Old Norse now; a friend of mine and me are about 3/4 through the New Introduction to Old Norse Reader.
I haven't encountered any detailed explanation for my first question, but the Reader's glossary at least translates both phrases.
"til at" is "in order to" (and therefore basically the same as "at")
and "fyrir innan" is "inside", as you said.
And about this, the grammar volume of the New Introduction to Old Norse says:
"Adverbs with the -an suffix combine with a preceding fyrir (cf. 3.7.1, 3.7.4) to form prepositional phrases indicating position relative to another (fixed) position".
and later also
"Apart from the above, there is a series of complex prepositions that trigger the accusative, made up of fyrir and a following adverb with the -an suffix (cf. 3.5.1). These indicate position relative to another (fixed) position, e.g. fyrir vestan hafit ‘west of the sea’, fyrir neðan kné ‘below the knee’ (further examples under 3.5.1). Sometimes the order fyrir + -an adverb may be reversed, but it should be noted that while, e.g., fyrir ofan always means ‘above’, ofan fyrir has two meanings: ‘above’ and ‘down past’ ‘down along’; in the latter sense it is not a complex preposition but a sequence of adverb + preposition (see 3.7.4, fyrir)."