While true, in your examples the third party is obtaining something that is already within their possession and they are not being asked to create something new. There is a reasonableness to searching for a document, but essentially Apple is being asked to create something that does not exist.

Think about the first court case where this "FBiOS" is used against a person that is on trial. Wouldn't the defense have a potential field day with this, and wouldn't Apple then be drug into every trial to testify? I could see the defense claiming that because the OS was replaced it is impossible to determine for sure what the original device contained versus what the government may have added to it.

Essentially from my very limited understanding of forensic techniques, techs are not allowed to ever add software to a device and can only search the device. Otherwise they are essentially "modifying" evidence. But I am not an expert in this area so I may not be fully understanding some of the intricacies.

That is so, but feds are not looking for evidence, but for operative information and leads.

Anyway - there will be enough developers willing to do that if feds can obtain the source code. The trick is apple to sign it.

Yea, it may be true for this one case that they are not looking for evidence so much as leads. The problem is this one case affects 100's or potentially thousands of others in the future, by setting legal precedent.

Based on news reports there are currently 100's of criminal cases and many prosecutors waiting to use the "FBiOS" once it is developed. If the DOJ's goal was strictly about terrorism there are alternative ways to solve this and Apple would likely be more accommodating, as it appears they have been in the past. But the DOJ is now looking to establish legal precedent within the courts so it can be used routinely and not just for terrorism cases.