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Not at all sure, but some of the Falcon 9/Heavy failures seem to suggest that SpaceX's thinking on landing is "don't give up, fly it all the way to the ground," when there's an issue.

Especially with the fact that Starship is eventually supposed to fly humans and doesn't have a limit on relights since the Raptors are electrically ignited, I would bet that the software will/does include the ability to light up extra engines in an emergency.