agreed that she is not supposed to force Frank,

and she didn't mean to, she just mistook him as one of her own kind and was just encouraging him to find himself. when she realized that she was wrong about him, all the way from the very start, she stopped. She realized that he had been himself, comfortably so.

Frank, as he himself put it, was "a little wise guy with a big mouth". He did lead her on to get her attention, so they both were responsible for the tragedy. Although it was too painful for both of them, so April chose to end it all, while Frank chose to blame all on her and live on with half a mind.

There was no hint on April's plan, actually. The piece that I quoted was basically all her thoughts before she carried out the last scene. But of course by doing that after the "safe" period, she knew she was risking her life. And by delaying her phone call to the hospital, she was making that risk almost unavoidable.

The main reason I read the book, was to find out why she didn't even try to get a divorce. Well, there was no hint about that. Rather there was more hints on the potential of the "state funny farm". I guess it's safe to say that April is not exactly a practical person, her suivival skills failed her at the end.