Neither North Korea or Iran have a ICBM that will reach the continental US. All the their current designs are IRBMs.
- Iran can reach Europe, so the NATO countries become their hostage against US intervention. The missile defense for Europe is to block their taking that course of action, or rather to make it an unreliable strategy.
- The best the latest generation of North Korean missiles can do is reach Hawaii, if they ever get them to work. But they have plenty of missiles that can reach Japan. Because of the feelings about Japan in the area from WWII and before, North Korea attacking Japan would be a bit like when Iraq attacked Israel during Desert Storm.
It doesn’t make any difference if an ICBM is liquid or solid fueled for purposes of intercepting it.
It makes a crucial difference. Solid fuel rockets are capable of more thrust, producing higher acceleration, thus reaching higher speed faster, reducing the already near-impossible response time.
If by “early phase’ you mean a boost phase interception with a missile, that is all but impossible due to the kinematics.
By early phase I mean the early part of the boost phase, while the velocities are still manageable. You are correct, it's all but impossible. However, it is the only phase where interception was concluded to be not fully impossible.
Mid course interception is practical if you can get early enough warning of a launch. That is the key – the sensor system. The satellite shootdown was against a target even faster than an ICBM, but it took 2 ships to do it because the missile had to be launched before the target was above the horizon in order to meet it. One ship to track the target ant the other to shoot it down.
Mid course interception is not just impractical, it's not possible. Just for the reasons you mentioned - not possible to create a sensor network that would be able to detect and track the target fast enough. Knocking out a satellite with a perfectly known trajectory is a relatively simple task.
Against Iranian missiles that tracking would be provided by the radar system in the Ukraine that Russia so detests. Against North Korea they deployed the Sea-based X-band Radar.
I am talking about US being the target, so Ukraine is not an option.
There are a lot of seemingly contradictory reports. The results depends on the assumptions behind the report. If the assumption was that they were trying to stop a major power (Russia or China) with an overwhelming number of missiles and no leakers permitted, then no, it will never work. Against North Korea or Iran with only a couple missiles, it is very possible to achieve greater than a 90% chance of success. It all depends on the assumptions.
Actually, it was a rather definitive report. It was a government requested comission made up of the elite academicians from the top US institutions, with the relevant expertise in the technologies involved. This report focused only on N. Korea and Iran as the targets. Sorry to disappoint you, but they concluded that it's all but impossible to defend against either country's ICBM launch.