After all, she had lost everything she had cared about - her mother, her parents, and her father. It made perfect sense.
As far as the ending goes, I don't think it's definite that they were eaten. It's vague enough that they have wiggle room to make a sequel. After all, they dropped the camera on the dock, and while we hear gun shots we don't have any definite proof that they were overwhelmed. There's no shouts or cries of pain or anything.
The only sort of nonsensical thing was their decision to leave the mall, but they didn't have much choice in the end.
I particularly liked how they showed the zombies spreading. The news footage was well done, and I liked how it made sense for Sarah Polley's character to be unaware of what was happening until it was too late.
And what the heck was up with that zombie head in the cooler? I have a feeling that the sequel will touch on that. From what I've read (which could be inaccurate) Romero's original vision for DAY OF THE DEAD, the action was set on an island where this sort of strange, post-zombie society had arisen. It had some elements of the filmed version of DAY, like the scientists experimenting on the zombies, but much of the plot was different.