1,I know John,than whom I have never seen a sillier man．这句话对

1.此句正确.
than/as用于比较时,从语法上来说视为连词,但是日常用法时却经常当做介词使用.
这两种用法都是可以的.
如：I‘m taller than he/him.I'm as tall as he/him.
此从句than whom I have never seen a sillier man并非比较状语从句,而是非限制性定语从句.
其中关系代词whom代指先行词John,将其代入从句中,为：
(than whom) I have never seen a sillier man
= I have never seen a sillier man (than whom)
= I have never seen a sillier man (than John)
如果定语从句的关系词在从句中充当介词的宾语,一般都会把该介词和关系代词一起提前至定语从句句首.
2.only less不是固定搭配,原因是less和more一样,用于比较级.
只是more是肯定说法“更...”,而less是否定说法“更不...”.
如：English is more important than Chinese in the world.
= Chinese is less important than English in the world.
如果用了more/less,后面的形容词/副词就必须用原级形式,因为这两个单词本身就是比较级的含义.
3.这个不能省略的.如果省略了,句意就不通顺了.
I can forget her no more than she（forgets）me.
和她不能忘记我一样,我也无法忘记她.
no more...than...是固定搭配,意思是“和...一样不...”.
4.此句than后面如果用their height,句意就无法理解了.
The windows are much wider than they are high.这些窗子的宽度比高度要大很多.
如果用：The windows are much wider than their height,则违背了比较必须是同类事物比较的原则,变成the windows和their height进行比较了,明显不对.
如果than后面的成分简化后不影响句意和比较对象,倒是可以的.
The windows are much wider than high.这种省略方法,个人表示赞同.
5.楼主理解正确,即：
No other men work as hard as does his brother.
= No other men work as hard as his brother does.
= No other men work as hard as his brother.
前两句把as视为连词,后一句把as视为介词.