Well, I do not need the most of these additional info, but for other may be of interest.
When I practice SMD soldering by plain iron station, first board I have made is 16-bit DAC (overkill for this purpose, but anyway I have planned it for experiments with playing WAV). I have read all related info before soldering about R-2R ladder, bought more than needed 1% 0805 resistors and before soldering tested each resistor value and pickup the closest ones - final results are in the specified range.
Thus, there is no question about precision of R-2R ladder. And there will be no need to cover all possible combinations (2^8), max 2 buttons push should be supported at the time.
I do not think it would be any problem with. Resolution of 10-bit ADC with 5V ref. is 5 / (2^10-1) = 4,887mV, which is 4 times more than need and certainly that is not 1%, but 400% total allowed error. With read voltage, it is trivial to determinate precisely which button combination was pressed.
Thanks to Daniel, this is now trivial.