MicrostockGroup

Possibly, the blue background might give the impression that it is.
I'm not sure on this - as always it comes down to the reviewer.

I'd pump it up a bit, give some more life.

Your copyright of course.

I agree with the above, the "white" paper looks a little bluish, it could probably use a nudge.

Underexposed rejections most likely are not based on the concept and setup but on histogram values.

Check the histogram on Luminance, your highest value is slightly below 230 and "white" paper is around 210. You certainly want those in the area of 240 without blowing out the highlight on the pen, of course. Protect the highlight with a mask and play around with levels and curves until you get to a satisfying distribution of tonal valuse.

You aren't using the low end of contrast as well, by the way. So check your post processing how to improve your contrast management.

Yep, agree with antoniodalbore

Both ends could be clipped a little more plus a little lighter in the highligts I think.

Here is my take attached (yours in the darker one ).  Each to his own of course (meaning, we will all have different opinions on what looks right)

I think you got the answers to your question, but I think its good as is.  But if you send it off to anywhere else you might lighten it just a little...don't want to make it look washed.