I have eyes. I see now. I had eyes yesterday. I saw then. During yesterday, I had seen then. If I had eyes, I could see now. If I had eyes, I could see then. I will have eyes. I will see later. I might have eyes. I might see later.

We saw them last week. is correct. but the first sentence is not correct after could you want to use the base verb (see) not the past participle (seen). You could see on their face. you can also say. You could see it on their face. I am not sure which one best fits the meaning you are looking for.

Yes that's correct