Murphy was a pessimist.
Again, I ask:
You will have to change one entry in "boards.txt" and then yes, you use the Arduino software.
What entry? I have CodingBadly's cores installed, do I need anything else?
What entry? I have your cores installed, do I need anything else?
Each board has an entry like this...
attiny84at16.upload.using=pololu
Some other examples that are known to work...
attiny84at16.upload.protocol=avrispv2
attiny84at16.upload.using=arduino:arduinoisp
You will have to change the "upload.using" entries for your programmer. I suggest waiting until you have all the pieces.
What's the chance of having the right address and legal commands being sent over?
I have no idea how the three protocols (SPI, I2C, Atmel programming) overlap. As I said, the Atmel folks are rather clever so I suspect that they designed the programming protocol to work reliably on an I2C bus.
Or I can just do a software I2C and have the LED on the SPI bus to prevent anything (ty Murphy).
I frequently program with LEDs on the SPI bus. Sometimes just so I know the programmer is making an attempt. So long as you can find a software I2C that meets your needs that should work.