Is it possible to do simple edits to transform a black and white SVG to a grayscale one?

Hope that helps 0.5 usually ends up loosing a lot of information unless the original image is extremely bright. In fact, any absolute threshold will mess up one kind of images or another.
A better method would be to make a histogram of luminosities and choose a threshold near the mode. This should work better on most images than any absolute threshold.

seems to work fine If what you're looking for is to threshold the image -- everything brighter than a certain value turns white, everything darker turns black, and you pick the value -- then a library like GPU Image will work for you.

Any of those help You can use Ghostscript's inkcov device to get color information about each PDF page. Here is an example command for a sample PDF (cmyk.pdf) of mine with its output:

gs -o - -sDEVICE=inkcov cmyk.pdf

   GPL Ghostscript 9.10 (2013-08-30)
   Processing pages 1 through 5.

   Page 1
    0.00000  0.00000  0.00000  0.02231 CMYK OK
   Page 2
    0.02360  0.02360  0.02360  0.02360 CMYK OK
   Page 3
    0.02525  0.02525  0.02525  0.00000 CMYK OK
   Page 4
    0.00000  0.00000  0.00000  0.01983 CMYK OK
   Page 5
    0.13274  0.13274  0.13274  0.03355 CMYK OK

gs -q -o - -sDEVICE=inkcov cmyk.pdf

    0.00000  0.00000  0.00000  0.02231 CMYK OK
    0.02360  0.02360  0.02360  0.02360 CMYK OK
    0.02525  0.02525  0.02525  0.00000 CMYK OK
    0.00000  0.00000  0.00000  0.01983 CMYK OK
    0.13274  0.13274  0.13274  0.03355 CMYK OK

gs                  \
  -o cmyk.pdf       \
  -sDEVICE=pdfwrite \
  -g5950x2105       \
  -c "/F1 {100 100 moveto /Helvetica findfont 42 scalefont setfont} def" \
  -c "F1                        (100% 'pure' black)    show showpage"    \
  -c "F1 .5 .5 .5   setrgbcolor  (50% 'rich' rgbgray)  show showpage"    \
  -c "F1 .5 .5 .5 0 setcmykcolor (50% 'rich' cmykgray) show showpage"    \
  -c "F1 .5         setgray      (50% 'pure' gray)     show showpage"    \
  -c "   1 0 0 0 setcmykcolor 100 130 64 64 rectfill"                    \
  -c "   0 1 0 0 setcmykcolor 200 130 64 64 rectfill"                    \
  -c "   0 0 1 0 setcmykcolor 300 130 64 64 rectfill"                    \
  -c "   0 0 0 1 setcmykcolor 400 130 64 64 rectfill"                    \
  -c "   0 1 1 0 setcmykcolor 100  30 64 64 rectfill"                    \
  -c "   1 0 1 0 setcmykcolor 200  30 64 64 rectfill"                    \
  -c "   1 1 0 0 setcmykcolor 300  30 64 64 rectfill"                    \
  -c "   1 1 1 0 setcmykcolor 400  30 64 64 rectfill        showpage"

seems to work fine Well that's a black and white output, its been halftoned to produce 1-bit output from 8-bit input. No matter what you do, if your device only supports 1-bit output then you're always going to have to apply some kind of screening to represent grayscale or colour.
You don't give any clues as to why ljetplus is the only device you can use. Without knowing why its going to be hard to offer any concrete advice.