老师们好，我最近遇到两道关于or的改错题，都错了，自己思考了很久，有一些疑惑始终解不开，老师们能帮忙解答我的疑问吗？

首先第一道题目：Sb. has become withdrawn or is lacking in motivation.

疑惑之处：这句话答案是“正确”，但是我觉得可以删去is，删去后，这句话貌似也说得通。

Sb. has become withdrawn or (Sb. has become) lacking in motivation.

Sb. has become withdrawn or (Sb.) is lacking in motivation.

这两句话貌似都说的通，请问不对在哪里呢？

第二道题：You will work on projects of varying size and complexity, or using some of the most up-to-date technology in existence.

疑惑之处：这句话错在“or”，我知道为什么，但是我纠结可不可保留or，通过修改其他地方来改正这句话呢？

我的修改：You will work on projects of varying size and complexity, or (you will) use some of the most up-to-date technology in existence.

这样改是不是也对呢，或者结构上至少是对的呢？还有一点疑问是，or前面什么时候可以使用逗号，什么时候不可以使用逗号呀。