Sometimes it is not apparent from looking whether the outer solid lamina of ABS has been thinned out by abrasion or not. I would go over the area that has been denuded of exterior vinyl pressing in firmly on the hull with your thumb. If the area feels spongy, the solid ABS stratum has been thinned out, and it won’t be long before it wears into the foam core. If not, you could just protect the exposed ABS by covering it with paint and reapplying as needed.
If the area is thinned out, it is definitely easier to apply reinforcement before the foam core is exposed. Either 6 ounce/square yard plain weave fiberglass (E or S 'glass) or 5 ounce/sq yd. Dynel will work well. If you want an abrasion plate with very minimal drag, you can feather the edges of the plate very nicely if you want to take the time.
I have never used KeelEasy. Some whitewater boaters I know who have used it tell me it does not hold up well for that use at all. The bond basically depends on contact cement, which will not result in as strong a bond as epoxy. I am not sure whether the edges of KeelEasy can be cleanly feathered or not. It is much thicker than a single layer of fabric and will result in more drag, although perhaps not enough to be noticeable.
When I add graphite powder to epoxy, which I have done often, I add it little by little and stir it in until the mixture is slightly thickened. I have never measured it be weight or volume.