I was wondering if forking could prevent a possible price manipulation of Bitcoin?
I really don't see how this could prevent it. Whoever is manipulating the price will move to this fork alongside the rest of the users.
As for Bitcoin's deflationary tendency in the long run, I don't know how this would help against price manipulation either.
Deflation would be pretty small, only due to people losing access to the bitcoins and the amount of bitcoins in circulation would stay pretty stable over time.
Bitcoin doesn't necessarily have to be the same as it was inteded. It is ok to innovate and it is ok to address any possible problems or concerns we didn't know about in the past. But discussion about this isn't really useful, since we couldn't stop it from evolving even if we wanted to.
There will always be forks and there will always be new implementations, we can only hope that majority will pick the best.