A few questions here, starting with parasite drag. Here, experimental measurements in a wind tunnel would be helpful, as the old standby graph of induced lift going down and parasite lift going up with speed is pretty generic. However, after looking at the PBY-3 Catalina and where they mounted the props (and flew it as a long distance patrol aircraft), one may conclude the lions share of drag is induced, particularly at lower GA speeds.
Potential energy (altitude, glider fuel) = m×gravity×height. One can see this increases linearly with weight. Speed increase needed to lift (at constant optimal AOA) a given weight increase is the SQUARE ROOT of the weight increase. Drag produced by this speed increase is speed SQUARED. This produces a linear relationship between gliding distance gained from increased weight compared with gliding distance lost from increased INDUCED drag.
Increased speed will produce a minor increase in parasite drag. So, efforts to quantify this would focus on the drag coefficient of the prop, fuselage and tail. Here you're in luck as all studies are done at optimal WING AOA.
Keep in mind Vbg is based on Indicated Airspeed, so one need not worry about pressure or altitude. So, for the practical GA pilot, Vbg will be fairly constant within the given safe weight range of your aircraft (but could be calculated while you do your gross weight and CG location work pre-flight), cargo carriers need consult the POH as Vbg will vary greater with a wider weight range.
Now, the other factor for Vbg is wind speed. Here greater weight is advantageous for wind penetration. Hence nature's inexorable millions of year process of trial and error has produced the heavy, fast high aspect ratio albatross to easily glide 1000s of miles on the wind. Glider pilots add ballast under windy conditions, while the lighter wing loaded, slower vulture delights in riding the thermals. Glider pilots may choose to lighten ballast under those conditions.
However, if wind is a factor, best glide slope (from best glide speed) can be determined just like in a landing, by observing how a fixed object in the distance moves in your windscreen. At Vbg it will rise the slowest. And landing is what you will be doing soon.