...does each sub combiner have to have the same voltage drop? Does the bus at the final combiner have to receive the exact same voltage from each sub combiner?

I prefer to think of it as voltage rise from the final combiner to the inverters.

It's basically the same calculation, except that your variables must be based on the assumption that the voltage at the transformer is the given. Don't use the nominal voltage of the inverter output and calculate a drop from that. Calculate the drop

*to* the transformer voltage from the inverter. The inverter will output at whatever voltage is necessary to push the current to the utility; the voltage at the inverters will rise too high if they voltage drop from the inverters to the transformer is too high. The busbars of the final combiner of course cannot be at more than one voltage to each other; that voltage will be determined by the interplay of all components.

Your engineering problem, which you need to avoid here, is that if voltage at the inverter terminals is too high they will shut off. If the transformer is higher than nominal, the voltage at the inverter terminals will be that voltage plus the voltage drop on the conductors in between. You are probably good with keeping all of your voltage drop/rise under 2%, but you may want to consult the utility on the engineering on their side. I have only dealt with this on much, much smaller systems where really I didn't have to worry about the effect of my system on the local grid.

I don't believe it's a problem to have different voltage drops for the different sub combiners but your concern is the maximum you can afford for any one of them.