But doesn't a business have to, at some point, meet a minimum level of efficiency to still be viable? Do subsidies and mandates make up for all that slack?
It's not just subsidies and mandates, it's large government contracts on maybe not actually needed systems. See the F-35 as an example of this, which has parts manufactured in a majority of states meaning it's impossible for congress to nix this sort of program because it would cost constituents jobs. Also how it makes it harder for congressmen and women to vote against these sorts of deals because it would cost their constituents future jobs.
I don't know about Airbus in particular but this is how it generally work with companies like Lockheed and Boeing.
Only if there's sufficient competition. Boeing does the same thing, spreading their components across most of the US (and into other countries) for the same reasons. It's an alternate to conventional lobbying. Instead of, "Hey Congressman here's $100,000 for your campaign." It's, "Hey Congressman, vote for aircraft X and I'll move to your state and create 2000 jobs. Don't, and I'll pull out the 2000 jobs that are already there."
I would think that building planes, as expensive as they are, would mean that you would want to sell them to everyone. In order to sell in a country, you probably have to commit to building (or assembling) some parts of it in that country. Breaking it up the way they did probably is as efficient as they can make it now. I guess we'll see in some years how that turns out financially for them.
I don't know about Airbus in particular but this is how it generally work with companies like Lockheed and Boeing.