Adding a 187KΩ in series with the ADC input will give you a voltage range of 0 to 61.38V.
Is the 60V maximum based on the battery voltage or the charging voltage? A 60V battery will have a charging voltage of around 72V in which case you will need to change the resistor value so that the ADC max voltage is higher than 72V.
GPIO 21 is passed through on the ADC Pi header. The only pins used by the ADC Pi are GPIO2 (SDA), GPIO3 (SCL), 3.3V, 5V and Ground. All other GPIO pins are passed through and can be used for other purposes.
You are correct that the battery positive will connect to R1 and R1 connects to the ADC input. The battery ground connects to the Raspberry Pi ground or the ADC Pi ground which are both connected together.
In continuous conversion mode the ADC samples the input continously and will have a sample ready to be read from the I2C bus. In one-time mode the ADC will sleep until you request a sample at which point it will wake up, take the sample and go back to sleep.
The main difference between the two modes is the power usage and the sample rate. In continuous coversion mode each ADC chip will consume around 135 μA and will be able to sample at the full speed for each bit rate. In one-time mode the ADC chip will use around 36 μA between samples but the sample rate will be slower depending on the bit rate. We have a page in our knowledge base that goes into detail on the different sample rates and how they compare in continous conversion and one-time mode.
In either continous conversion or one-time mode the power consumption from the ADC Pi will be minimal compared to the power used by the Raspberry Pi. If you are only going to use the Raspberry Pi to monitor the battery voltage then you may be better off using a lower power platform like an Arduino which will use a fraction of the power that a Raspberry Pi uses.