Three problems : while you're trying to stream missiles in slowly, the opposition is going to mass launch against your silos, cutting off your stream.

Nukes produce broadband noise. Improved radars let you get much better signal to noise and see through it anyway.

And the warheads you're streaming in need to actually detonate to give noise. Otherwise you just get a debris field.

A debris field of sufficient size is also noisy. MIRVs can have a couple decoy warheads whose job it is is to just release debris to create radar noise.

Theoretically, yes. But that is part of the point of improving the signal processing. And there's 4-5 different radars looking at stuff coming over the pole at Fylingdales, Thule, Clear, Cavalier, possibly at sea, depending.

I believe this dynamic is exactly what MRVs were designed for. One missile with numerous war heads, arriving on target one after another in rapid succession (independent targeting came later.)

There are some videos of MIRV reentries on youtube that really something to see. Seeing those bright lights come down from the sky in a line, one after another, is simultaneously chilling and awe inspiring: https://www.youtube.com/watch?v=j7X89a531CY

What kind of smoke screen is that? Are interceptors radar guided?

Yes, incoming warheads would be tracked by ground radar, with guidance beamed to the interceptor.