Learn Icelandic: Er endurgerðin að öllu leyti upprunaleg? - Is the restoration in every respect original?

This follows the fixed pattern of að + dative neuter adjective + leyti. The neuter noun leyti means an extent or a respect, and it takes the dative case after the preposition að. Here, we insert öllu, which is the dative neuter form of allur (all/every). Literally, it means at all extent, or in every respect.

Can I just say alveg instead of að öllu leyti here?

You could absolutely say Er endurgerðin alveg upprunaleg? to mean Is the restoration completely original? However, að öllu leyti is slightly more formal and carries the specific nuance of in every single respect, which is perfect for discussing the meticulous details of a historical restoration.

How does the noun endurgerðin break down?

It is a compound of the prefix endur- (re- or again) and the feminine noun gerð (a making, model, or type, derived from the verb gera). With the feminine definite article -in attached at the end, it literally means the re-making, which translates here as the restoration or the remake.

Why does the adjective upprunaleg not have a visible grammatical ending in this sentence?