There's no way that's a reliable way to dispose of a body, especially with a hand still sticking out...
Nope. Purely done for the dramatic reveal. I would have liked it if his death was shown, and how she got from point A (his house...) to point B (dumping part of his remains in a river)...and for her to have more fun with it!
Imagine:
If I was her, I would have played along long enough to lure him into neutral territory (a hot spring), under the guise of a lover's get away and then drown him. Then probably smashed the side of his head in to look like he slipped and hit his head on the rocks (or vise versa, which ever is the quietest, I mean, you are at a hot spring, wouldn't want to disturb the other patreons). If she plays devoted/loving girlfriend in front of everyone and he dies, authorities can chalk it up as an accidental. Why would they suspect a couple so in love to suspect anything otherwise? And bouns: no body to clean up! I mean, people have accidentally killed themselves in dumber ways I'm sure.
Though, she isn't great with keeping her emotions in check...
And there I go again, rambling (sigh). Should really stop reading manga 2 in the morning...