Good morning Elizabeth,
This is an excellent question and one that I actually hear quite often. Let’s look at some the variables in play to see how you might change the scenario more to your liking.
First, let’s examine the graphite that you are toning your surface with. Most graphite vehicles (wooden pencils, woodless pencils, graphite sticks, etc…) have hardness ratings that will tell us something about how the material will behave in a number of contexts.
Most graphite pencils manufactured today are rated according to either a European or an American hardness grading scale. The European system is often attributed to an early 20th-century pencil manufacturer while the American system is attributed to both Nicholas Conté and 19th-century pencil maker John Thoreau.
The European system uses a 20-step scale that employs the numbers 2 through 9 along with the letters H, B, and F to communicate the material’s clay to graphite ratio. A graphite rod with a high clay content results in a harder material (H) while a rod with a lower clay content will be softer, allowing for a greater range of “blackness” (B). The (F) designation, which is near the center of the hardness scale, is often thought to mean “fine point”—however, some argue that the letter was employed by manufacturers arbitrarily. The European scale ranges from the hardest (9H) to the softest (9B), with grades H, F, HB, and B populating the middle of the scale.
The American system is a much smaller scale with numbered steps ranging from 1 to 4. Higher numbers are used to indicate harder graphite rods. The two systems can be aligned at the following scale points: 1 with B, 2 with HB, 2.5 with F, 3 with H and 4 with 2H.
To understand how one graphite hardness grade might provide an advantage over another we will need to consider our drawing surface, method of application, and desired finish. Rougher drawing surfaces can typically accept and hold more loosely-bound, soft material while smoother surfaces can facilitate the fine marks of well-bound, harder material without the need for surface “bite.” Softer graphite will provide a greater value range potential at the cost of a greater tooth requirement and a quickly eroding point. Harder graphite requires less tooth and can better maintain a point. However, these clay-rich grades yield a more narrow value range and an increased potential for drawing surface damage if used with a heavy hand.
What this means for you specifically Elizabeth is that the harder graphite (material with higher binder content), will in general, be far more difficult to remove (achieving the bright specular highlights often seen with glass) than the softer material (less binder content). One alternative is to use powdered graphite with a brush or “sachet.” The powdered version will not have the binder found with the rod delivery systems ((pencils, sticks, etc.). Additionally, in case you are not familiar with it, a sachet is a small porous bag or packet containing a material intended to interact with its atmosphere (often populated with a scented or mark-making material). Here is an example of artist Casey Baugh using powdered charcoal with a sachet with a procedure that may be somewhat similar to your own:
I should state here that EdgePro’s sachet is over $50.00. That seems really pricey, so know that you can make your own from just about any porous material. Here’s one example of a homemade sachet that probably landed in the $1 to $5 range.
(taken from Pam’s Cool Stuff for Starving Artists, http://www.pamscoolstuff.com/2015/08/messin-around-with-charcoal.html)
Switching to a powdered toning material (something with little to no binder, or even a softer overall material like charcoal) may attack the eraser problem as well. With less binder, a material can be lifted more easily so that your removal options may drastically improve.
It is true that you may be able to increase your contrast by starting with a darker tone—but remember, if your binder content is high then you may be increasing your inability to remove the material as well (ultimately carrying the contrast ceiling from one key to another.)
Another option for increased contrast, in addition to lifting material, may be to employ another tool—like a white pastel pencil or stick. This is a long-standing common practice that you may have seen with some artwork when its description contains “heightened with white.”
I hope some of these ideas and insights help. An additive /subtractive dynamic with dry media is a fun process and I know how frustrating it can be when the materials don’t seem to be in agreement with how one feels they should behave (still happens to me from time to time.) 
