Scalia was hoping mad about the Obamacare decision that, among other things was an attempt by the state to force a citizen to buy a product. I won't rehash the arguments, they are well known, but the short answer is that Scalia would likely be opposed to forcing a company to build something. If the 'something' existed already, there might be a different argument to make considering that there isn't a 4th Amendment question (as the perpetrator is dead.) The idea of the state to compel an entity to build something (which is the same as forcing them to buy something as they must spend money buying the labor to build it,) would be, in Scalia's view, against the Constitution.

The plaintiffs would have to prove with substantial weight that there is a national security issue (i.e. An imminent threat.) That would be hard to prove as the government is really just investigating and fishing as opposed to having proof there's a nuclear launch code on the phone. I would suspect the 9th Circuit would rule in favor of Apple.. As such a 4-4 Supreme Court would by default result in the 9th Court decision standing.