But the question is, did the FBI read that image and seek a court order anyway?

The FBI is slow, but not incompetent by any means. This was a terrorist's cell phone, a crucial link to determining whether this was a lone act or a coordinated plan.

Even if they couldn't introduce it in court, wouldn't they do it anyway? In fact, it seems very suspicious that the FBI suddenly found an exploit vendor after Apple refused to bow to governmental pressure.

So the question is becomes, "If they had the data anyway, why seek a court order?"

To examine the problem, consider that to the FBI, accessing a single phone is a win in a single battle. That is not what the FBI is after.

What they are after is total victory. They believe only an ironclad legal precedent can win their war. They may be right. They may be wrong. I strong suspect that they are wrong.

Encryption is like the tide. It can be pushed back, but in the end it will win.

Water always finds the cracks.

The FBI were looking for an easy solution that every law enforcement can use and that is to force Apple and other device makers to unlock the devices or undermine their security sufficiently for traditional mobile forensic approaches to be viable.

The FBI didn't find an exploit they found a vendor with an exploit, likely a vendor with an exploit that did not involve doing irreversible damage to a phone to extract the data from it. Forensics and even digital forensics have pretty strict rules to what counts for a forensically secure data extraction and what doesn't. When in doubt you simply wait for a better method to come around, you preserve evidence this is why we can go back and reexamine evidence with new techniques, the first question of any new forensic process is "does it alter the state of the evidence".