OK. Servers might be decentralized, but if we decided we wanted something changed, could we distribute a patch to 51% of the network and fork the code? If no, it isn't decentralized in the slightest.
Yes, we will be able to do it 
https://ripple.com/wiki/Consensus
As we haven't seen the code you cannot know that, and at the moment you need to trust a single central agency to have told you the truth about ripple's implementation.
Even if they aren't malicious, you need to trust they succeeded at security by obscurity, and the system won't exhibit critical flaws when (or rather if) open sourced. The chances are slim, as any expert will tell you.