You can not clean the "sponge" until it is free of stain or color. What you want to look for is "gunk", "gel", deposits of linty ink build-up, etc. Yours is clean enough. The yellow residual stain should disappear after a short while. I am surprised it is still there if you are making linearization checks, etc... There can only be so much residual yellow left in the print head that normal printing or head cleans will not rid it. You may not have wiped the ink stems well enough with a q-tip and some yellow pigment entered the shade 7 cart...that remains a possibility. If the yellow staining in the highlights does not go away - you may need to empty the yellow cart. But, this is rare.
The linearization is just a little darker and only in the mid tones. When we see this we usually think workflow related. Are you on Mac or PC?
If Mac, can you tell us your workflow in relation to
a) embedded Gamma
b) version Mac OS X
c) printing from Print Tool, Photoshop, or other (what?)