Etydhv

I'm a bit confused when trying to understand the second line in the below.

To add a bit of context to help, the main character has traveled back in time, so that he can stop something bad from happening in the future that involves these two characters and their friends. In the few sentences before this he accidentally says something that he shouldn't have known, but he does since he learnt it before he traveled back in time (no one else knows he has gone back in time).

My first bit of confusion stems from the usage of くれる. My understanding of くれる is that it is used when someone other than the speaker does something for the speaker or someone close to them. So in this case the only logical answer is 咲良, however I am not quite sure. The second part that I am not sure of is what is being referred to by その状態.

If I had to guess what is being said in the second sentence I think the main character is saying that it would be for the best if 咲良 does not know anything about what has/is going to happen.

（やっぱり咲良と接する時は、なるべく何も知らない振りをしよう……）

何も知らないままでいてくれるなら、その状態が一番いい。

Yes, the subject of 何も知らない is 咲良, and this くれる is used because 咲良's ignorance is beneficial to the speaker. Of course 咲良 is doing nothing intentional or visible for the speaker, but since he is feeling 咲良's ignorance is desirable and thank-worthy, くれる is still a natural choice. その状態 also refers to the fact that 咲良 knows nothing.

何も知らないままでいてくれるなら、その状態が一番いい。

If 咲良 stays unaware of everything, that (situation) is the best.

Your guess is correct. He appends the くれる because he is "thankful" to Sakura for staying the way she is (and hopes that she does). 「その状態」 here most likely refers to her state of ignorance, i.e.「何も知らないままの状態が一番いい」.

To be 100% precise, it would actually refer to 「何も知らないままでいてくれる状態」. Or, alternatively, you could interpret the sentence as implying「何も知らないままでいてくれるなら、何も知らないままの状態が一番いい。（いてくれないなら、そうでもない）」. But I'm sure most would interpret it the more straightforward way, as I assume the author intended, despite the grammatical ambiguity (or rather, flaws) of the sentence.