英语翻译For in his late poetry there is a rather crotchety perso

for in his late poetry there is a rather crotchety persona,这是for引导的原因状语从句,也是定语从句的主句（你给的好像是半句话,因为没有跟原因状语从句对应的主句）.into whose carpet slippers some ambitious young man with a technique as accomplished could slip是定语从句.
whose引导定语从句,修饰先行词persona,（因为定语从句一般紧跟它所修饰的先行词）把它放到后边可以看得更清楚,some ambitious young man with a technique as accomplished could slip into whose carpet slippers,其中some ambitious young man with a technique as accomplished是主语,could slip是谓语动词,whose carpet slippers作为into的宾语.
slip into carpet slippers我猜想是由于这个短语：fill one's shoes,意思是令人满意的替代,slip into穿上,网上可以搜到Auden和carpet slippers的很多网页,摘一段来看：Famous were the carpet slippers which Auden regularly wore in his American years,in which he traveled,lectured,and recited poems.可见作者故意使用了这个典故.
as这里的用法,我不确定,as可用作副词,介词,连词,语法上副词as后跟形容词accomplished是说得通的.因为accomplish是及物动词,as后边并没有be动词,所以as这里不太可能引导从句,也有可能是省略了主语和be动词,但这样的话,就相当于as the young men were accomplished,这个意思不好,accomplished还是修饰technique比较合理,你给出的汉语翻译也是这样理解的.

进入地毯上的拖鞋
如同煮熟的鸭子也可以飞掉，就是获得的成就也能不慎失去一样。

进入谁的地毯拖鞋之内””当做完成会滑倒