Echtelion is the traditionalist. Erik would do as it please him, as he did. He's a Duke, why he must think he must obey elections for Avamar despite what he wants for his Duchy? That's another power struggle, one with the system itself. That's why he prefer to appoint his nobles.
He never had anything personal against Eldarion until Rhiannon ask her region back. Why a womanizer would ignore the appeal of a Lady in detriment of an unknown man? When he refused the request and forced to Erik to send an order enforcing his position and his desire over his lands, there's the beginning of a power struggle. And yes, she is a political supporter. Everyone is his duchy are, since they are appointed, mainly for Tabost and Linbar, regions that belonged to Sirion and changed alliance to him. To keep these regions, he always select very well who will be appointed. If you care or not, he have a preference for Ladies, another reason to prefer her instead of him. That's to change a political supporter for someone he never received a word about. Another power struggle would be with Celine, since he failed to make Perdan obey the prisoner agreements to release Rhiannon in time to be reappointed.
Erik faced a major confrontation with Zakilevo when he was PM and tried to reform the Duchies. When he said he would do it Erik wanting it or not and that he would talk with his Ladies and Lords, Erik did the same, requesting his Lord to stay at his side and ignore the PM. Everything in Avamar is about power struggle.
While they never hated Eldarion, he was furious when he ignored his request. It must be silly, but Erik would always prefer a good Lady.
