So, good news and bad news. Good news is that you can replace the embedded JPEG with ExifTool (as shown in Can I embed existing JPEG's into DNG's as a preview?). Or, you can use
dcraw -e to simply extract the preview to look at it separately.
The bad news is: this is extremely unlikely that Preview will fall back to doing its own demosaicing / RAW conversion. That would be a lot of work for a weird corner case, since all mainstream cameras produce RAW files with embedded previews. (That is: I am very sure of the premise of the question: Preview doesn't do conversion. It just shows the embedded preview.)
I would suggest using something other than Preview to do your comparisons. RawTherapee would be my choice for free software for OS X. That way, you're really comparing similar fruit, rather than being at the mercy of whatever processing is done to create the preview.
It's important to realize that there is no single "see the true RAW pixels" in a meaningful way. There needs to be some interpretation. The closest you are going to get is probably from FastRawViewer (proprietary software with a free trial for Mac and Windows), which presents the data with minimal interpretation. Or, you can use
dcraw -h to get a quick and dirty conversion. But, I'm not really sure of the value of this; it's more practically useful to use a full program (whether RawTherapee or Lightroom or Darktable) to make the best conversion you can for each, and then compare those.
It's also very possible that the images from your Canon simply are better in the particular areas you are examining. Despite similar sensor size, the dedicated camera has the advantage of a much larger lens, and there's plenty of other factors that could come into play as well.