The ending plan was actually an evolution of the surrounding circumstances. We know from Tony's detective work that Killian's original test runs of Extremis had resulted in catastrophic failure. There were a few unexplained explosions that were similar to the Mandarin's attacks (temperatures reaching a specific magnitude, 3000 Centigrade) but were not attributed to the Mandarin.
These early test runs were, in large part, ignored by Killian and he didn't see the need to attempt to cover them up. After all, Killian doesn't send his agents to cover up information on that early test failure until Act 2. It wasn't until the test runs continued to fail and no fixes to the Extremis were foreseeable that Killian began to panic. He realized that, eventually, the government would discover that he'd been failing to deliver on Extremis in a rather... explosive manner.
This lead to his creation of the Mandarin. As a terrorist, the Mandarin would be expected to initiate attacks on the US and any future Extremis incidents could be attributed to him. This way, the US government wouldn't even think about a local US research & development firm being the actual cause of the explosions. After all, an upcoming terrorist is taking credit for them, and given the current War on Terror, who would blame them?
The conclusion is more speculation on my part, but as things escalated, Killian likely realized that there was no way he could cover up Extremis forever. At least, not without some grand spectacle to distract everyone. Obviously, the death of the President on live television would be attention-grabbing enough and it was just lucky coincidence (for Killian) that the vice-president's daughter had a disability. With that, Killian realized that he could control the presidency. The plan that Killian eventually arrived at was born more of coincidence and desperation than actual preparation.