They way in which I understand it is that as soon as the server source code will be made open source, the whole network will completely become decentralized because everybody can run their own Ripple servers/nodes/gateways.
Correct. If we, for example, change our server to do something nefarious, it would just be ignored just like someone changing their Bitcoin client to send transactions spending Bitcoins they don't have.
I dont know what you mean by 'central bank keys' though, I guess you are referring to the vast amounts of Ripples (XRP) that they will still be holding? Their stated pla is to distribute those over the course of the next years (decades?) to help solve the initial distribution problem. If at any point in time regulators would threaten to seize those assets (XRP), they could simply move them to an unknown address or distribute them all at once in a faucet like fashion as they have done in this forum already.
One could imagine a scenario where those keys get into the hands of some group bent on destroying the Ripple network. It's hard to imagine what they could do though other than release them in large quantities and cause the price of XRP to drop. But that wouldn't harm Ripple's use as a payment network. In fact, it would just make it cheaper to use.
And, of course, the community of Ripple server operators could react with any technical changes needed. In principle, they can make any change whose advantages outweigh the disadvantages in the eyes of the majority of major validators.