Not so sure about
A specialized ballistic missile defense platform based on a commercial ship hull —
The US military has historically preferred to intercept ballistic missiles outside
the atmosphere. The advantage is that one missile defense battery can cover a
very wide area. A specialized ballistic missile defense ship could be kept farther
back from more forward groups, protecting them without giving away its position
with easily detectable radar emissions.
https://www.files.ethz.ch/isn/22265/17_Simple_Model_Calculat...
part of the reason why the US has BMD ships is that they can placed in places such that the footprint works since the ocean covers like 75% of the Earth's area. To really be out of range of aircraft, anti-ship missiles and all that you'd have to be hundreds of miles away from the threat and that could well put you out of the footprint. Not to say that you couldn't have clever answers such as the launch vehicle being separated by the radar though most BMD systems use track-with-missile guidance that require the missile be in close communication with the radar for the terminal phase.
This partly gets into the reason why exoatmospheric intercept is preferred. In order to maximize the useful coverage envelope, they need to maximize missile speed. Unfortunately, there is a speed threshold beyond which endoatmospheric terminal guidance becomes extremely challenging due to limitations imposed by material physics. If your terminal guidance is exoatmospheric, you can mostly avoid those issues.
That said, it is clear that the US has been leaning heavily into moving more defense to airborne missile carriers. For example, the SM-6 can now be launched directly from F-18s instead of destroyer VLS cells, which greatly extends the potential range of ballistic missile defense coverage. The B-21 Raider, while it can carry bombs, is essentially an extremely stealthy missile launcher with a very long range and loiter time.
Relatedly: if you park an AEGIS BMD ship off the coast of San Francisco, how far inland can they defend with SM-3 against an ICBM? Approximately eastern Kansas. Same question, but Hampton Roads: approximately western Kansas.
The footprint defended by SM-3 is actually fairly huge.
Footprint charts here:
https://mostlymissiledefense.com/2016/06/30/strategic-capabi...
what I find suspicious though was up until 2000 or so there was a doctrine that midcourse intercept was unworkable because ICBMs could be supplemented with a large number (hundreds) of lightweight decoys that would be impossible to discriminate from a real warhead. That led to systems like Sprint that would wait until the warhead was re-entering the atmosphere and easy to discriminate because it was not slowed down so much by the atmosphere, doesn't burn up, etc. I dunno if the sensors really got better or if they just decided to go ahead anyway for political reasons.
The terminal guidance sensors could pretty trivially discriminate decoys since the 1980s, when they stopped using radar guidance and switched to broad-spectrum imagers. The decoys really only worked on radar. A decoy capable of spoofing the imagers would have to be so large, complex, and heavy that it would be tantamount to adding another warhead which defeats the purpose.
This is why they stopped testing against decoys. They had tons of test data that the terminal guidance systems could reliably discriminate them so it didn’t add anything to the tests other than cost.
This source would argue the other way
https://cissm.umd.edu/sites/default/files/2019-08/2000-UCS-C...
It seems that a warhead or decoy would fill a single pixel on the sensor until you got very close to the target, 1 km or so, at which case you have to execute a high-g maneuver in a few ms. The state of the art is a "two color" system where you could make either the warhead or the decoys any "color" you want with surface treatments and/or thermal management (worst case: put the warhead inside a shroud cooled to liquid nitrogen temperature and fire the weapon at night when it won't be illuminated by the sun.)
There was a test in the early 2000s I read about where they were able to pick out a warhead which was intermediate in properties with a set of decoys with various surface treatments. That 's great but they knew exactly what they were up against which we wouldn't know if it was a North Korean missile.
I'd have more faith in the discrimination abilities of ground-based radars in the 12 GHz range than in the "two color" focal plane imager system.
This gets deep into the classified domain, no one is going to be talking about it here. :)