Memento can use any camera app that supports Android intent functionality. When you select a photo, you can select the camera app. When you select a camera app, it can be set as the default. You can always clear the default application by Android Setting: Settings > app > default camera -app > open by default > clear defaults.
The photo file name is what it is, based on the date & time. There is no other option. You could campaign for that to change at memento.UserVoice.com.