https://nap.nationalacademies.org/read/13189/chapter/4?term=...
Your question about delta-V is answered in that same report a few pages above.
That’s how you read “in principle” and “in theory”? The cost of launch is used, in the third bullet, to justify not analysing the idea further. Not as the “only…limitation.”
As your source says, even a working space-based boost-phase interceptor is trivially defeat-able with “primitive” ASAT capabilities.
Brilliant pebbles are a fucked concept. You’re citing a no-math NAS paper for good reason—it’s good to speculate about it in case we learn something new about how gravity works. Barring that, it’s well-recognised nonsense.