AFAIK (and exeng/et al will either back me up or call me out on it
), you won't need to worry about the TinyWireM I2C, as a renamed copy was included as Wire, so it *should* be as near as a drop in substitute for that used on the Nanos/Unos etc.
Since the MPR121 uses I2C, any code referencing Serial is either using it for debugging / output / linking to Processing. So the question here is are you using some example code that uses Processing (which you can do away with), or are you developing something that needs to be able to talk to processing via Serial?
For the soft-serial stuff, have a look at the example provided with the Digispark Arduino IDE package - File -> Examples -> DigisparkSoftSerial -> SoftSerialExaample
But basically you'd include the libraries
Create a soft serial object declaring which pins you'll be using (maybe 1 & 3 in your case?)
SoftSerial mySerial(1, 3); // RX, TX
In the setup function, initialize the soft serial, specifying baud rate (just like normal serial)
And then use it normally in the main loop... i.e
newInput = mySerial.read();