With apologies to readers who have no desire to get so far down into the weeds ...
Yes, Jason, of course I realize that the Court will probably not reach the severability question unless it first finds that the amended Section 5000A imposes a mandate to purchase insurance and that such a mandate is unconstitutional. Which is one reason I've argued that the Court should simply hold that 5000A doesn't mandate insurance maintenance and be done with it. (It might be possible for the Court to reach the severability question without resolving the 5000A question (the avoidance canon and all that), but I agree that the most likely sequence is that the Court will consider 5000A first.)
It does not follow, however, that the 2017 Congress would have assumed that Section 5000A would coerce or even induce anyone to purchase unwanted insurance. Every member of that Congress--especially the Republicans who sponsored and approved the zeroing out of the Shared Responsibility Payment--realized, as did everyone else, that five Justices of the Court had just concluded that Congress lacks the constitutional authority to enact a mandate to maintain insurance. Therefore even if some consumers would -- unreasonably -- read Section 5000A to mandate purchase of insurance, it'd be absurd for any of them to actually purchase insurance based upon that reading -- just as no one feels compelled to comply with the dozens/hundreds of statutes that remain "on the books" today after the Court has declared them to be unconstitutional.
All of which is to say that, whatever one's views might be on the best reading of Section 5000A, no one in 2017 could reasonably have believed that the amendment would cause anyone to purchase insurance--and no one in Congress did think that. And yet Congress retained all the rest of the ACA, anyway--a fact that, as Abbe argued, is sufficient to find that the rest is severable.