Difference between 最高领导人 and 主要领导人之一?
党和国家领导人 is both. It can refer to a few people or just someone. For example, 党和国家领导人某某(和某某)出席了某个活动。
最高领导人 can be translated as supreme leader. There must be only one at one time. And 主要领导人之一 is just some one important.  For example, only 5 最高领导人 in PRC history. But anyone in the Political Bureau is one of 主要领导人。 


The address(称呼) of important people of CCP is significant and subtle. Similar address may have big difference. Here is a great Chinese post analysing how to call CCP leaders after their passing, the best I've ever read.