No, what you mean to say as long as your modified software adheres to the rules of the protocol set by the developers of Bitcoin-Qt/Bitcoind after they push a protocol change that is approved by Gavin Andresen, who has the final say. Ask him. He wants to be the Linus Torvalds of Bitcoin. He wants the approval process of Bitcoin changes to be just like Linux.
If the majority of the network accepts Bitcoin-Qt/Bitcoind protocol changes under Mr. Andresen's guidance, your modification will mean squat. It will fork and you will be left with your toy Bitcoin that nobody accepts.
Yea but if nobody agree with his protocol change, no one will follow his fork anyway and instead will continue to use the old fork.
In theory. This is dependent on good communication throughout the whole community. If people don't know about the changes, they have nothing to go against. They will accept the upgrade out of apathy.
The chance of this occurring is increased when we have a central release. Competing releases resolve this in many ways.