Why would "Zȳha kepa kostōbe tale sylvie trēsī ēza" not be correct?
Why doesn't sylvie become sylvii here in accusativ ?
Because it's "sylvie" if the noun is solar/lunar and in the singular, and "sylvii" if the noun is solar/lunar and in the plural for both the nominative and the accusative.
What's the difference between "zyha", "jevi" and "aoha"? Are there more translations for "your"?
zyha is his/her (if the noun it's referring to is solar or lunar), "jevi" is "you (plural)" and "aoha" is "you (singular)".