^The problem is the persons who knew about the conspiracy b/n USNA and Gue Jie are either under USNA jurisdiction or death. I don't think Raymond would go so far as to leak that info to Japan sides.
I still try simulate the situation which may spark the new conflict b/n Japan and USNA otherwise we won't be able to see Lina appearing again. Only Tatsuya'real identity as Mahesvara being exposed may force the stars to move, i couldn't think of other reasons.
It's possibly though that Lina may fight along side Tatsuya as allies against the last boss. However, considering the next villain is someone who used to provide supports to Tatsuya, Lina joining is not likely going to happen.
For the villains of the next arc, many guess he should be Katsuto. I personally bet for Karzama(under the forced order or duress). Katsuto is too righteous to coax for condemning Yotsuba or to pitch him against Tatsuya. Others 10MC are to weak for being a serious threats, even Saegusa now is being weaken due to Koichi's blunder. If Koichi want to entice others to do his bidder, who would believe him due to his low credibility. Koudo also is out of the picture. Not to mentions Maya's influences over other head of 10MC(in good ways from Tat's past deeds).
(edited) there is spoiler saying that that someone appears in name only and it will be a large scales battle so i guess Aoba then.