The spacing variable is derived from the blockchain's parameters and the actual state of the chain, so it's a consensus rule. If any node goes rogue and decides to use a different value as part of its block generating procedure, other nodes will simply not accept its blocks, like with any other violation of the consensus rules.
We don't have any more detailed write-up of the MultiChain consensus process at the moment, though it's on our roadmap to do this in future. But you'd do well to read about how bitcoin works, since MultiChain is quite similar structurally, but with proof-of-work replaced by mining diversity. Forks can occur just like in bitcoin, and are handled in exactly the same way, although the mining-turnover parameter lets you tune things to make them unlikely, to optimize performance.
