By default there is no such concept as contract owner. Such concept exists only if it has been coded into the contract.
So it depends on what kind of functionality the contract has.
If by killing you mean selfdestruct, then the selfdestruct function requires an address parameter which specifies to which address all remaining Ethers are sent.
Gnosis Safe + Ownable
This is the easiest and most robust way. The fees on Gnosis Safe 2.0 are not that massive, as you pay only once at the transaction execution.
Gas fees as what they are and they are outside of your control.
For the problem given, a simple governance contract would be the best, where there should be a function to propose (only by owners), vote (only by owners), and execute/cancel (only by owners).
Execution of proposal itself should be very simple, checking if proposal is already executed, and execution of submitted target, value, method signature, and calldata.
As you noticed, there is no way to send a single from multiple addresses. There is always just one sender.
For Ether transfers the Ether value is always taken from the sender's balance and you can't take it from other accounts (that would be a rather big security hole). You can't take them in one transaction even if you have their private keys. But what you ...