I have a few questions regarding the 12MHz oscillator...
Is it possible to use an oscillator of frequency <12MHz?
Is it possible to use the AVR's internal oscillator?
If so, what changes would need to be made to the firmware, and are these changes easily made?
12MHz Oscillator Substitute
The oscillator frequency must be 12 MHz +/- 0.2%. This precision is usually only met by a crystal resonator. Even a ceramic resonator may not be precise enough.
The precision is computed as the number of bits in one transaction (~ 100 bits) and the maximum bit skew of 1/4 bit until the end of the transaction. That makes 1/400 bit times or ~ 0.25%.
In practice, the clock should be as precise as possible.
You may succeed with a calibrated internal RC oscillator, but I really doubt that you can meet the requirement above over the entire temperature range, voltage range and life time of the device.
The precision is computed as the number of bits in one transaction (~ 100 bits) and the maximum bit skew of 1/4 bit until the end of the transaction. That makes 1/400 bit times or ~ 0.25%.
In practice, the clock should be as precise as possible.
You may succeed with a calibrated internal RC oscillator, but I really doubt that you can meet the requirement above over the entire temperature range, voltage range and life time of the device.