The “Send to Compressor” command does not use your sequence render files (your lower third is a rendered effect), but rather renders directly into the codec you are exporting to.
This can slightly improve quality when exporting to MPEG-2 for a DVD, but in this case it appears to be tripping you up, as it is not accounting for HDV’s anamorphic frame size correctly, in addition to the resize to 720p. I think thats why your graphics look stretched.
Best to take the extra step and export a Reference Movie, drag that into Compressor and see how it goes. Reference movies only take a few seconds to export.