Our first concern is the use of the epoxy. Although you have may had success with this process previously, we do not recommend the use of Enduro-Var with epoxy or any oil based products. Enduro-Var works well on its own or with GFs water based finishes. There is a great possibility that future finishes can fail.
Nor do we recommend adding denatured alcohol, which should not be used in any of General Finishes water based products. Denatured alcohol can change the polarity of a finish formula, kicking out polymer particles that would look like grit on the surface. Denatured alcohol could also deactivate the defoamers in the formula.
The use of Extender won't help you. It is a water based product and will not melt through existing layers of finish in the same manner that solvent finishes do.
We interpret that you are trying to remove witness lines which are basically sanding marks. For your current situation, we recommend attempting to build the area with just Enduro-Var, then buffing out with a 2000-3000 grit. You may have to sand down and start over if that is not successful.
Future forward. Use Enduro-Var on its own. Although it is a favorite among Luthiers, it is a unique hybrid finish that does not pay well with other finishes.