Thank you, glad you're taking a look at it.
1. You'll need to build an adapter with a TinyCircuits proto board so that you can connect a programmer- or connect to the TinyDuino and use ArduinoISP software. Then you need a software build environment- there are a lot of tutorials for setting up tools like AVR-GCC, avrdude, make, etc (WinAVR if you're on Windows), especially for 'raw' coding of a standard Arduino, and they should have about the same effect. However, the avrdude config file does need an addition for the ATtiny841- let me know if you get this far, or need help getting there.
2. For a multimeter you'll probably need a resistor/capacitor filter to get a nice voltage output. I've also remembered that the default behavior of the library is to let the motor 'coast'- ie, if one pin is being driven low, the other pin alternates between driven high and undriven- not driven high or driven low, like you would want for a voltage output. Since you're looking into the library, you should be able to write your own setMotor function to use 'maxPWM' instead of zero for the non-pwm pin. ie, a line like 'writeCommand(COMMAND_MOTOR_1,0,abs(val));' becomes 'writeCommand(COMMAND_MOTOR_1,maxPWM,abs(val));'
3. DRV8837 datasheet claims 280 milliohm total(high side plus low side) on resistance at 800mA, which would be 0.224V, so that may be what's going on if your motor is drawing over 1 amp. Is that the case?
Ben