As to hard fork - it must be implemented by all network actors, not only miners. The blockchain must be altered. So miners are only a part of network entities that must actively support it.
Gavin wrote the hard fork for Parity client, Vitalik takes care about Python implementation. Yes, these are different languages aimed to implement the fork. There must be also Golang implementation and some others.
The fork is implemented by changing clients of (ideally) all network members to versions that include the fork code. This assures that the blockchain stored locally by full nodes is affected in exactly the same way globally.
I saw this step by step analysis. I did similar on my own.