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Can Eirys opt not to move and get an aiming bonus on her Phantom Seeker bolt to shoot something out of LOS?

Look at the rules. If it's a ranged attack, then the answer is
yes. If it's a magic attack, the answer is no.

According to Prime, under Crossbow it says "Eiryss may
fire one of the following following bolts per attack". Since
the Crossbow is her ranged weapon, it counts as a ranged
attack and so the answer is yes.

The rules are usually about reading the text specifically,
though in a few cases you need to refer to FAQs and Errata.