Canon XA10 - some basic problems

Hi folks, perhaps you can help.

I have an XA10. I'm using a mac and have a program called clipwrap to bring the files from MTS to Prores.

The camera iteslf offers a function of downconverting HD footage to SD footage, but the letterbox footage is then squeezed into the 4/3 aspect ration.

Why do they do this? How do I get the footage in an SD format - ideally by cropping the sides instead of squeezing it?

Also the file format is a bit strange - the video and audio are separate files. I would prefer a single QT file.

So again the question is - how to get HD letterbox footage to SD NTSC 4/3 Prores.
Thanks
DAN MCGUIRE