It should be possible to define that you need e.g. 90% of the shareholders to approve a change to the contract. The 90% could still steal money from the 10%.
It is. The example code for a DAO [0] illustrates how everything from minimum quorum for proposals to margin of votes for a majority is configurable. However like you said, so long as the contract is mutable, it's possible for a majority to subvert it.
[0]: https://www.ethereum.org/dao