'every breath you take' spoiler question

I know what happened and who did it. And i get what happened to one of the stalkers.

But what i don't get is at the end when we find the step daughter killed the cop there were a couple things i couldn't figure out.

First how did the body turn up in the river after a year? Did they ever explain that? Did it just float up from somewhere or did someone move the body and throw it in the river a year later?

Did the cop help the stepdaughter dispose of the body? If not, how did she do it by herself. She looked too tiny to be able to move a dead body?

If he did help her, didn't he wonder why she did it? He seemed surprised at the end to find out that she was in love with him.

When you stop and think, can you forget to start again?