If it takes a vast majority of participants to agree, then not really. It's decentralized, not 100% immutable. Will get harder as it gets larger but at this stage it is good to still have the ability.
At this stage yes. That's why I don't hold ether as being solid enough yet to be a store of value, aside from the fact that it's being inflated at ~25% pa. Needs to go through these teething problems now while that is still the case.
Why would that be different at later stages? Bitcoin grew more centralized with time, not decentralized: The miners with the most powerful equipment also have the most ability to become even more powerful.
It's not that simple. In the end it's what users actually use that matters. Mining on a fork that sees no use would be suicidal. So miners need to follow users, or their investment is for nothing. And users tend to follow developers, at least until there is a contentious fork.
So while there is no clear single party that makes decisions, active developers have the most say.
