I was hoping to use EasyTransfer as I assumed I would be able to more easily change data types and sizes without having to bit-bang the changes through... perhaps I'm over thinking!
This isn't wrong, EasyTransfer makes this a little bit easier. The question is, does this approach make it easier for you in your project.
I suppose I need an I2C primer which is more in-depth than the Arduino references.
Try the introduction of Nick Gammon (a moderator on this forum), it's the only one I know which is Arduino based.
EasyTransfer (imho) still feels like the right, more dynamic solution for communicating to the web-server slave... do you foresee any problems using them together?
No, other than it might guide you again to think about the I2C as a fast version of an UART communication. It was designed to let a processor talk to some devices (chips) and not to let two processors talk to each other. Although this is possible with I2C, it forces you to select one side to be the master while the other side is the slave. In a processor-processor communication it's not always obvious which one should be the master.