What can "modified" mean when describing an ingredient?
Modified on the label literally means... well... modified, that is changed or altered.

I am unable to find a Standard Of Identity formally for "modified starch."

Modified starches are typically modified in a number of ways (see the linked Wikipedia article for the full detail), but the most common one in instant mixes is to pre-gelatinize the starch.  This permits it to more instantly dissolve and thicken items, even without being heated.  

Your description of the filling thickener makes it sound like the starch is in fact modified for low temperature thickening.



In the case of your pie glaze, reverse engineering an industrial product which consists largely entirely of chemicals that are not easily obtained for home use may be... erm... fruitless.

You would be better off describing the outcome of the glaze, and trying to create a home recipe to come close.  You are likely to get a good result, perhaps even better than the commercial product.

Fruit pie fillings are are typically starch thickened, during the baking process.  However, you want an "instant" pie which is not cooked.  No-bake fruit pies are fairly unusual.  You may get better results by making a more traditional baked fruit pie.



This remainder of this answer is educated speculation, but I have not tried it.

To recreate this affect at home, you want a stove top filling method.  Sugar, tapioca starch (which tends to give a transparent effect, and is not sensitive to acid), lemon juice (for flavor and tartness, replacing the citric acid in your mix), and salt, perhaps with some spices should come very close.  

Baking911 suggests running instant tapioca in your food processor to get a finer powder, that dissolves more easily.  Note that instant tapioca is in fact modified, as it is has been gelled, and then dried&mdash;so it may be close to what you are looking for.

You can bring this mixture to a low simmer, with your fruit, and then pour it into your shell, and it should do fairly well, although you will have to find the right proportions.