1.) Add this line to the master retroarch.cfg:
video_aspect_ratio = 1.25
2.)Not sure, probably a setting for this you can add to the retroarch.cfg as well.
3.)You can change the master alsa sound so it is lower. In the terminal type the below command and move the slider down on the screen that pops up to where you want it.
4.)No, Super Mario All-Stars/Super Mario-AllStars + Super Mario World do not work. Not sure about Super Mario Bros. for the NES. Maybe try a different rom rip?