If I stay over at Nanami's house, we can save the trouble of having Nanami come over to our house.
So even if I have to go back home to go to her house, it's fine.
ご質問ありがとうございます。
"If I stay over at Nanami's house,"=「ななみさんの家でお泊りするならば、」
"we can save the trouble of having Nanami come over to our house."=「ななみさんが家に来る手間が省ける。」
"So even if I have to go back home to go to her house,"=「だから、もし一度に私が家に帰って、その後ななんみさんの家に行くとしても」
"it's fine."=「それで構わない。」
ご参考になると幸いです。