This is a really interesting idea! A couple of things you might want to consider for a future version: I notice you are dividing the charge product by the distance between charges, but Coulomb's law actually uses the square of the distance - this is important because the attractive or repulsive force decreases very rapidly with distance.

The structure.GetDistance function that foldit Lua provides gives you the distance between the alpha carbons. You might get better accuracy by locating the polar atoms themselves and adding a band between them, then asking for the length of the band. That would definitely make the recipe much slower, though. One way to limit the time increase would be to query alpha carbon distance first (which is quite fast), and only use a band to get a more accurate distance if the alpha carbon distance is below some threshold (close enough that the difference between alpha carbon distance and sidechain to sidechain distance is expected to be significant).

I hope you'll keep working on this!

Coulomb's law for the force between 2 charges does go 
as 1/r^2, where r is the distance between the 2 charges,
but the potential energy due to this force goes as 1/r.

The gravitational force between 2 masses also goes
as 1/r^2 while the gravitational potential energy
goes as 1/r. 

The equations for electrostatic force & potential energy
are very similar to the equations for gravitational force &
potential energy. One difference is that the electrostatic
ones go as the product of 2 charges while the gravitational
ones go as the product of 2 masses.

Because of these things, I think using 1/r when calculating
the Binding Energy DDG makes more sense than using 1/r^2.

https://en.wikipedia.org/wiki/Inverse-square_law#Electrostatics
https://en.wikipedia.org/wiki/Inverse-square_law#Gravitation
https://en.wikipedia.org/wiki/Gravitational_energy#Newtonian_mechanics
https://en.wikipedia.org/wiki/Electric_potential_energy#Electrostatic_potential_energy_of_one_point_charge

I notice the recipe attempts to keep track of the top three pairs contributing to repulsion, and the top three pairs contributing to attraction. When a new pair exceeds the #1 spot in these lists, it gets inserted at the top of the list and the others are pushed down. However, if a new pair falls between #1 and #2, or between #2 and #3, it is not getting added into the list as it should.

The recipe is using the following values for the point charges (measured in e, elementary charge) for each amino acid:
0 a, c, f, g, i, l, m, p, v
1 k, w
3 r
-1 h, n, q, s, t, y
-2 d, e
I assume these are based on the number of polar atoms in the sidechain. D and E are commonly considered charged, while I guess the polar atoms on the others could be considered to have a partial charge. Is there a table somewhere of how charged the various amino acids are? On those that have both a partial positive and a partial negative, should these point charges be considered separately rather than aggregated?

I tested it at puzzle Sadbox 1892 with Binder metrics and I find 2 times your result:
Calculatr: -16 kCal/mol
Foldit= -32.55 kCal/mol

Hope this helps