making作动词enjoyed的宾语（enjoy doing）
bring用原形是因为这个短语所致：make sb do sth
buying是因为before这个介词,介词后面的动词要用ing形式.
是的the dress后面省略了that/which,因为that/which作宾语,所以可以省略.
she enjoyed herself making the assistant bring almost everything in the window
主语 谓语宾语宾补 宾语宾补 宾语定语
before finally buying the dress she had first asked for
时间状语介词宾语 定语从句（she前省略了that/which）
主句使用过去式是因为发生在过去,与从句没有先后关系.
很高兴为你解答!
老师祝你学习进步!
望采纳,多谢你的问题!^_^非常感谢老师您细致的回答，有些思路了，但是还有一点就是：……before finally buying the dress she had first asked for最终还是买了先前看好的那件衣服。（先前看好： 用了 had first asked 是过去完成时？）4、before finally 前面的为什么不用过去完成时，或者过去进行时？had asked for发生在buying之前，所以这里用过去完成时其实before buying相当于before she bought的意思。