问题一：only 置于所修饰的词语之后

根据《柯林斯高级英语词典》，将 only 置于所修饰的词语之后主要是为了强调，该词典的相关解释如下：

When only is used as an adverb, its position in the sentence depends on the word or phrase it applies to. If only applies to the subject of a clause, you put it in front of the subject. Only strong characters can make such decisions. Otherwise, you normally put it in front of the verb, after the first auxiliary, or after the verb be. I only want my son back, that is all.. He had only agreed to see me because we had met before... I was only able to wash four times in 66 days. However, some people think it is more correct to put only directly in front of the word or phrase it applies to. This is the best position if you want to be quite clear or emphatic. It applies only to passengers carrying British passports... She'd done it only because it was necessary. For extra emphasis, you can put only after the word or phrase it applies to. The event will be for women only... I'll say this once and once only.

英汉版《柯林斯高级英语词典》对以上文字的翻译如下：

only 用作副词时，其在句中的位置取决于它所修饰的单词或短语。若 only 修饰句子的主语，则置于主语前，如：

Only strong characters can make such decisions. 只有坚强的人才能作出这样的决定。

除去此种情况之外，only 通常置于动词前、第一个助动词或 be 动词之后，如：

I only want my son back, that is all. 我只想要回我儿子，仅此而已。

He had only agreed to see me because we had met before. 他同意见我只是因为我们以前见过面。

I was only able to wash four times in 66 days. 我66天才做到洗了4次澡。

但有人认为将 only 直接置于它所修饰的单词或短语之前更恰当，这样可以表达得更清楚或更具强调意味：

It applies only to passengers carrying British passports. 这仅适于持英国护照的乘客。

She'd done it only because it was necessary. 只是出于必要她才这样做的。

若要进一步加强语气，可将only置于它所修饰的单词或短语之后：

The event will be for women only. 该活动仅限女性参加。

I'll say this once and once only. 这件事我就说一次，而且仅此一次。

问题二：为什么only做副词不单单能修饰 v 和 adj 还能修饰 n了呢?

副词原则上不用于修饰名词，但有少数副词可以，有的语法学家称之为“焦点副词”，即“集中注意”时用的副词，如even, just, merely, only, really和simply之类的副词可以用在其所修饰的词之前，以使人对其集中注意。

关于焦点副词的用法，可参考英语语法网的以下文章：

http://www.yygrammar.com/Article/200802/100.html 

http://www.yygrammar.com/Article/201510/4240.html