I’m not sure why Miwa was unwilling to tell Saeko the whole story. It’s not as if she has anything to hide.
In the first place, Miwa knows that Saeko has been acting weird when she hangs out with other people (even if Miwa isn't certain that it's jealousy), so telling Saeko that she just exchanged contacts with her old crush, who even invited her to a hotel, probably seems like a bad idea to her.
More significantly, it doesn't seem like Miwa ever properly got over Shiho, so the fact that she showed up and suddenly wanted to be close with her again has Miwa reexamining her own feelings. If she tells Saeko everything that she and Shiho talked about, she'll have to try to reassure Saeko that there's nothing between them, which isn't something Miwa can say with total certainty. It's easier and more forgivable to lie by omission than to lie directly.
That "If you love me, kiss me" part is a microcosm of everything wrong with their relationship; they're trying to use their physical connection to compensate for their lack of trust and a mental/emotional connection.