Q1: Yes, I think those three sentences using active voice are correct, including the second one. And they seem natural, too.
Q2: Yes, we can definitely use passive voice in relative clauses.
男の子は女の子にペンを借りました。
This sentence can be (at least technically) rewritten using passive voice in two ways:
- A: ペンは女の子から男の子に借りられました。 (with ペン as the subject)
- B: 女の子は男の子にペンを借りられました。 (with 女の子 as the subject)
The sentence A may seem tricky because I suddenly used から instead of に. "By X" can be translated as "Xに", but 「ペンは女の子に男の子に借りられました」 is obviously strange, so I had to rethink the particle choice. Do you know that, in the active version, we can also say 「男の子は女の子からペンを借りました」? Knowing this, you can make the former version straightforwardly.
The sentence B is often called 迷惑の受け身, and it implies the girl is bothered by the situation (i.e., she didn't want the pen borrowed). Actually this sentence is not very good because it happens to be ambiguous and can also mean "A girl could borrow a pen from a boy". (借りられる also means "can borrow", as you know) If there's enough context, this won't be a large problem, and we can quickly fix this by using the subsidiary verb しまう to clarify the feeling of 迷惑.
- B2: 女の子は男の子にペンを借りられてしまいました。
I think most people would say this sentence is natural, although the 迷惑 feeling is added which was not in the original active version.
Now we have the correct passive sentences, we can construct relative clauses from them. With the sentence A:
ペンは女の子から男の子に借りられました。
→ 女の子から男の子に借りられたペンは高いです。
Although understandable, this sentence is unnecessarily complicated, and we don't usually say this. Stick to the active version 「男の子が女の子[に/から]借りたペンは高いです」 unless you have a good reason.
And with the sentence B:
女の子は男の子にペンを借りられました。
→ 女の子が男の子に借りられたペンは高いです。
→ 女の子が男の子に借りられてしまったペンは高いです。
I think the last sentence is natural if 迷惑の受け身 is intended.