I can't think of any way in which the FBI's incompetence would have legal implications for Apple's obligations. How would that work?
If they're able to help, and can be compelled legally, why would the FBI having previously had the ability to get the data but losing it to a mistake change their obligation?
The point is that the legality of the court order compelling Apple does not depend on whether or not the FBI made a mistake in other investigative approaches. If the order is illegal, then it doesn't matter whether the FBI needs the access it provides or not, it's still illegal. On the other hand, if the order is legal, then the FBI needing it due to a mistake doesn't suddenly make the order illegal.
Not the same. This is more like if the FBI wanted access to investigate a space murder and demanded Virgin Galactic to build a special shuttle to allow the FBI's equipment to the station, something only Virgin Galactic can do because they're the only ones who have the key to the space station.
and Space Station Technical services had a key to the space station but the FBI ordered them to throw it away some time before contacting Virgin Galactic and saying put people to work building us this special shuttle because otherwise we can't get in.
Then let's say the FBI handled a confiscated SSD drive wrongly, and some of the communication circuits of one of the flash chips holding the data got fried as a result. Your only bet to get data off that chip is the manufacturer of the chip. Could the FBI compel them to try?
But that might affect the reasonableness of the request. If the FBI can do it with off the shelf tools, it's less reasonable to ask someone else to do it than if there's no other option.
If they're able to help, and can be compelled legally, why would the FBI having previously had the ability to get the data but losing it to a mistake change their obligation?