They would still have to deal with delays that might incur. I imagine that it might even be possible to create an image of all data on the phone, then after it erases the data, just upload the backup and try again. However, all those measures raise the time needed for brute forcing to unacceptable levels. It could probably be done, but at an enormous cost of money, time and effort.
Again, this is not a backdoor. What they are asking for is more akin to telling Apple to remove metal plates from the front gate so they can ram through it. Nothing is being compromised except that particular phone. Sure, if they can do with that one, they can do so with any other one, but they'd have to physically acquire it first. Generally, it's common sense to consider any device that you don't have exclusive physical access to to be potentially compromised. Any device that is seized, stolen or lost is compromised, period. Also, this firmware would be useless for any phone but this particular one, because even though it can be modified to run on another one, Apple would have to re-sign that new version (as the signing system's entire point is to prevent unauthorized alterations like that).
There is no way to misuse software written specifically for a single device, one that they already have in their possession, to boot. It's hardly likely for them to misuse even the ability to force Apple to do this, because in the end, they'd still need physical access to the device in question. They might have been heavy-handed with wire tapping, but actually seizing someone's belongings is a much rarer occurrence.