There's no ICBM by the time any realistic interception scenario takes place. There's a slightly diverging cloud of warheads, chaff, jammers, light (inflatable metallized film) and heavy (small thingies that produce plasma in the higher atmosphere) decoys. The interception of a single target is hard because of what you said, the interception of any realistic salvo is impossible due to the sheer volume of incoming stuff.
Nope, the MIRVing/decoy deployment is done right after the boost phase, that way you need the least fuel to achieve the same target spread. That's some 5 minutes into the flight and mere hundreds of miles from the launch site. If it's Russia you're talking about, the missiles (or rather what's left of them) are still over their territory, some 200km up.
