Aegir is the type that leads by doing things himself. Though he has proper education and is not stupid, he performs much better as a force of nature rather than controlling troops. So Celia acting as his deputy really helps offloading that burden
Pretty sure Aesir rekted a country for enslaving the elves before... even if that's too long ago for human, it would be stupid to disregard a disaster if they wage war with elves again.