Question ステーキのお店なのですが、

[Theseus]

Context:
今日は仕事が終わってから、友達と夜ご飯を食べに行きました。ステーキのお店なのですが、食べ放題のパンも美味しかったです。食べ過ぎました。

I guess "のです"+"が" mean "although" or just "and then" here, and "～のです/～んです" expresses some kind of emphasizing, and the sentence pattern is "名詞だな＋のだ（です）".

(1) "な" in "お店なのです" is "連体形" of "だ", is it right?
(2) May I ask why the "な" is necessary for the sentence structure? What is the difference between "ステーキのお店ですが" and "ステーキのお店なのですが" ?

 

[Toritoribe]

"な" in "お店なのです" is "連体形" of "だ", is it right?

For the sake of convenience, yes.

May I ask why the "な" is necessary for the sentence structure?

It's the explanatory の. This の functions as a noun, so な is necessary to connect お店 and の.

What is the difference between "ステーキのお店ですが" and "ステーキのお店なのですが" ?

The difference is subtle, but の gives a contradictory nuance more strongly, just like ステーキのお店なのに.