Technically this can be done via a standard softfork.
No it cannot. Clients running the old rules are not able to use Bitcoin as they have normally. They are unable to use Bitcoin at all because they cannot see the transaction data in the blockchain and thus won't know about confirmations. This proposal is not backwards compatible, and backwards compatibility is the key point of a soft fork.
The same is true for any softfork. For example, if some portion of the hashpower were passionately against the OP_CLOCKTIMEVERIFY fork, they could create a competing softfork that makes any block including a OP_CLOCKTIMEVERIFY transaction invalid. If this occurred, then Bitcoin will split into two competing cryptocurrencies (both softforks of the original chain).
They could, but that is also why we wait for a supermajority of miners say that they are willing to enforce the new rules before actually enforcing.
Yes, in the same way a standard softfork uses the majority hash power to attack any chain under the old rules.
No it does not. A soft fork simply lets the old chain die out if people are still mining there. They aren't actively attacking the old chain as your proposal suggests. A normal soft fork would still allow miners to mine on and extend the old chain. This causes problems, see the July 4th forking incident for an example of this.