It should be known that Ned and Vaemond are NOT the same. They wanted to expose bastards but Ned was doing to support the true heir, and Vaemond was doing it for his own empowerment. Viserys set the rule of inheritance regardless of gender, so even though Luc is a bastard, that means Baela is heir.

the king can choose an heir.

jaehaerys already arbitrarily chose his 2nd son baelon (vizzy's dad) as his heir over rhaenys (his oldest son aemon's only child) 10 years before the council, which incredibly pissed off several houses and his wife queen alysanne because it goes against andal tradition of daughters before brothers.

only when baelon also died a decade later did he decide to defer the choice to a council, and only because there were mumblings of a potential civil war starting.

house lords can also choose heirs, it's just rarely done. walder frey threatens to skip all his older children for his youngest one in the original series, as an example.

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