Load Testing Modules: Should all modules be the same voltage before measuring voltage drop?

No, the voltage of each module being approximately the same isn't critical to doing a load test.

Instead it would be better to discharge each module followed by a charge of each module to the same mAh input. This can be done on the hobby charger with a single D->C (discharge, charge) and setting the input limit. So try to calculate the average of all modules capacity, then set input mAh limit on the charge cycle to 2/3 * average capacity. So guessing around 3000 mAh or just do the calculation.

No, ignore the self-discharge of modules and only use internal resistance for pairing. Ignore self-discharge of modules and only use capacity for deciding which ones to get rid of and replace in the pack.

Internal-resistance is a better measure of the health of the module that can be measured instantly. rather than waiting a long time for a module to reach a lower voltage. Think about what happens when the modules are being charged, they are being charged series and voltage is being monitored in pairs of modules. If each pair had vastly different internal resistances, each pair would get out of whack from each other pair in terms of total pair voltage, faster when being charged.

No not that I am aware of. Just buy used modules and do all testing and sorting yourself. Anyone taking the time to do all this testing is probably doing the work for putting into their own pack, not to resell them. Some sellers could possibly put in something in description like "good for core deposit return only" meaning they tested the modules and they are worthless unusable low capacity.

 

That's fine, just want to pick a value that has all the modules at the same or very close state of charge.

It's fine to compare the no-load voltage with load voltage. If you happen to have a second load, and calculated internal resistance using that 2nd load, that would be interesting to see if it differs.

The main thing is to perform a balance charge before putting the modules in the pack. That would reduce the chance of getting a code right away. Balance charge means putting all the modules in the pack with the + polarity side all facing the same direction. Then use a wire to connect all the + terminals and another wire to connect all the - terminals, then sit like that for 24-48 hours.

First pick all the modules that are going into the pack, based on the best mAh capacity. Remove all low capacity modules not being used.
Enter all the internal resistances into a spreadsheet, then sort from low to high. Keep matching the lowest IR module with the highest IR module of the remaining modules, until all are paired. The goal is to have the total IR of each pair' be approximately the same value. The secondary goal when pairing is to have each pair's total capacity to be approximately the same value. Also locate the pairs that have better capacity towards the center of the pack and the worse capacity towards the edge of the pack. The reason being is the heat building up in the center of the pack causes capacity reduction over time and doing it this way the pack will equalize over time and last longer.

It's also unlikely to throw any codes initially because everything about the HV battery is forgotten and it goes into some sort of "learning" mode for a while. At least, that's what I've read that other people posted saying they kept unplugging their 12V battery to temporarily get past a HV battery code and keep driving short distances.