ETH would be the most constant token for all cases, and obviously the cheapest. It’s not only about paying gas in ERC20, but about contracts paying execution in ETH (or ERC20), so keys from a multisig wouldn’t need ether for authorization of execution.
But the most powerful point is the user experience for dapps that use another token, so users don’t need to take care of 2 balances for interacting with any ethereum based Dapp.
Ether would be always the best for execution, and ERC20 would be a convenient slightly more expansive way to transact in any Dapp.
For your questions, 2 and 3 depends on the token. 1 is a.
At the end, if you need to convert (sell) your tokens to get ETH to pay for the gas, ultimately 1 would become b, because the amount of gas required for the conversation of ERC20 could get more expansive, unless the user always do it in big batches.