the freedictionary引用Random House Kernerman Webster's College Dictionary的内容，对代词what的释义之一为：the kind of thing or person that。也就是说，当what引导名词性关系分句而指人的时候，what指的是the kind of person(s) that, 而不是the person(s) who.

你的以上二个句子中的what从句都是名词性关系分句。插入语的解释是讲不通的。第一，插入语必须用逗号与句子其他部分分隔。第二，插入语不影响句子意思。而以上二个句子中并没有逗号分隔，而且去掉what we call或what is called，句子意思是不同的。你的二个句子可以分别作如下理解：

1 Most of what we call geniuses are successful only because they have made extraordinary efforts.

= Most of the kinds of people that we call geniuses are successful only because they have made extraordinary efforts.

2 They are what is called "the lowly" in capitalist society.

= They are the kind of person that is called "the lowly" in capitalist society.

关于what引导名词性关系分句时主句和从句谓语动词数的问题，可以参读the freedictionary对what这种用法的说明：

Usage Note: When what is the subject of a clause, it takes a singular verb if the word or phrase that completes the sentence (the complement) is singular, as in I see what seems to be a dead tree. It is plural if a plural noun or noun phrase completes the sentence, as in He sometimes makes what seem to be gestures of reconciliation. · Clauses with what as either subject or object may themselves be the subject of a sentence, and sometimes it is difficult to decide whether the verb of the main clause should be singular or plural. When the what in the what-clause is the object of the verb and the complement of the main clause is singular, the main verb is always singular: What they wanted was a home of their own; when the complement of the main sentence is plural, the verb is most often plural: What American education needs are smaller classes, though one also encounters sentences such as What the candidate gave the audience was the same old empty promises. When what is the subject of a what-clause that is the subject of a main clause, there is greater variation in usage. When the verb of the what-clause and the complement of the main clause are both plural or both singular, the number of the verb of the main clause generally agrees with them. When the verb in the what-clause is singular and the complement in the main clause is plural, one finds both singular and plural verbs being used. Sentences similar to both of the following are found in respected writers: What drives me crazy is her frequent tantrums; What bothers him are the discrepancies in their accounts. When the complement of the main clause consists of two or more nouns, the verb of the main clause is generally singular if the nouns are singular and plural if they are plural: What pleases the voters is his honesty and his willingness to take on difficult issues; On entering the harbor what first meet the eye are luxurious yachts and colorful villas. Occasionally the choice of a singular or plural verb may be used to convey a difference in meaning. In the sentence What excite him most are money and power, the implication is that money and power are separable goals; in What excites him most is money and power, the implication is that money and power are inextricably bound together. 

【1】Most 【主语】// of what we call geniuses【后置定语】//  are【系动词】//  successful 【表语】// only【强调状语】//  because they have made extraordinary efforts【原因状语】// . 

参考译文：我们称之为天才的那些人中的大多数人之所以成功，只是因为他们付出了非凡的努力。

备注(1):most of+名词 意思“...中的大多人” 介词of 表示部分与整体之间的关系

备注(2):what we call geniuses结构为call sb+名词=把某人称作为，what we call geniuses为名词性关系分句。What=the persons who 所以what we call geniuses=the persons who we call geniuses(我们称作为天才的那些人).属于带有后置修饰语名词短语/名词词组。关系代词who代替先行词the persons并在定语从句作谓语动词call的宾语，geniuses为宾语补足语，结构为call the persons +geniuses (把这些/那些人称作为天才)

备注(3):are successful(很成功)为系表结构

备注(4):only(仅仅/只是)为副词作状语表示“强调”

备注(5):because they have made extraordinary efforts. 为连词because引导原因状语从句

【2】 They【主语】//  are【系动词】//  what is called "the lowly" in capitalist society【表语从句】// .

参考译文：他们是在资本主义社会中被称作的“底层”。

备注(1):What is called “the lowly”in capitalist society为what引导名词性关系分句在主句作表语或者说叫“表语从句”

备注(2):what为融合型关系代词用法what=the thing that

备注(3):what is called “the lowly”in capitalist society=the thing that is called “the lowly”in capitalist society，“the lowly”in capitalist society作表语从句中的主语补足语。主动结构call sth+名词(把某人称作为某物) 被动结构sth be called +名词(某人被称作为某物)

充当融合型关系代词使用what的其先行词可以表示“物/事/人/数量/时间/地点更多例句如下

【1】表示“……的东西或事情”：what=the thing that

They've done what they can to help her. 他们已经尽力帮助了她。

He saves what he earns. 他赚多少，积蓄多少。

What Mary is is the secretary. 玛丽的职务是秘书。

【2】表示“……的人或的样子” what=the person that/who

He is no longer what he was. 他已经不是以前的那个样子。

He is what is generally called a traitor. 他就是人们通常所说的叛徒。

Who is it that has made Fred what he is now? 是谁把弗雷德培养成现在这个样子的？

【3】表示“……的数量或数目”what=the number that/ the amount that

Our income is now double what it was ten years ago.

我们现在的收入是10年前的两倍。

The number of the students in our school is ten times what it was before liberation.

现在我校学生的数量是解放前的10倍。

【4】表示“……的时间” what=the time that

After what seemed like hours he came out with a bitter smile. 似乎过了几个小时他才苦笑着出来。

The young girl was too frightened to speak, standing there for what seemed like hours. 小女孩吓得不敢说话，在那儿站了几个小时。

【5】表示“……的地方” what=the place that

This is what they call Salt Lake City. 这就是他们称为盐湖城的地方。

In 1492, Columbus reached what is now called America. 1492年哥伦布到达了现在所称的美洲大陆。