Is a relative clause a legit interpretation here?


For full context, see here:
<https://www.docdroid.net/hK45eJm/img-20170916-0001-new.pdf>

The sentence in question is taken from line 14:

>
> [内ヶ嶋]{うちがしま}[為氏]{ためうじ}とその[一族]{いちぞく}で、[白川郷]{しらかわごう}に[帰雲城]{かえりくもじょう}という[城]{しろ}を[造]{つく}った。  
>  => "It was Tameuji Uchigashima and this lineage who built the castle called
> Kaerikumo."

I interpreted the で after その一族 as the continuative form of copula です and
aligned its tempus with the 造った.




Let me include the previous sentence:

> しかし、15世紀後半に、ここにやってきて、住み着いた者がいる。  
>  内ヶ嶋為氏とその一族で、白川郷に帰雲城という城を造った。

It seems like the subjects (主語) are left out in the sentence in question:

> （それは/彼らは）内ヶ嶋為氏とその一族で、（彼らは）白川郷に帰雲城という城を造った。

「それ/彼ら」 refers to 「15世紀後半に、ここにやってきて、住み着いた者」("people who came here and settled
down in the late 15th century") in the previous sentence.

So it'd be literally like:

> It was / They were Tameuji Uchigashima and his family, and they built a
> castle called Kaerikumo-jo in Shirakawago.

You're right that the で after その一族 is the continuative form of a copula. So,
grammatically speaking, you could split the sentence into two, like this:

> それは、内ヶ嶋為氏とその一族だ（った）。（そして）彼らは、白川郷に帰雲城という城を造った。

...but I don't think a relative clause is involved here.




Let me include the previous sentence:

> しかし、15世紀後半に、ここにやってきて、住み着いた者がいる。  
>  内ヶ嶋為氏とその一族で、白川郷に帰雲城という城を造った。

It seems like the subjects (主語) are left out in the sentence in question:

> （それは/彼らは）内ヶ嶋為氏とその一族で、（彼らは）白川郷に帰雲城という城を造った。

「それ/彼ら」 refers to 「15世紀後半に、ここにやってきて、住み着いた者」("people who came here and settled
down in the late 15th century") in the previous sentence.

So it'd be literally like:

> It was / They were Tameuji Uchigashima and his family, and they built a
> castle called Kaerikumo-jo in Shirakawago.

You're right that the で after その一族 is the continuative form of a copula. So,
grammatically speaking, you could split the sentence into two, like this:

> それは、内ヶ嶋為氏とその一族だ（った）。（そして）彼らは、白川郷に帰雲城という城を造った。

...but I don't think a relative clause is involved here.

