1.我借给室友的东西他从来不摆 在/回 原来的地方。The original sentence is basically correct, however, if you use “回” instead, it will be more clear and coherent.
2. 你把盖子盖上 ，要不菜炖得特别慢。
From the context, easily we can see that the cover belongs to the wok, and it should be put on the wok, there has not other extra choice.Therefor you can cut “锅” out to make the sentence more fluency.
3.我想请你帮助我收 葡萄 。
Just more brief and smooth.