In theory, it could help. In practice, for KYC to reduce spam and scam calls, FCC would have to be willing to drop hammer big time on people and telcos who allow it to happen. With current political climate in the US, I don't see that happening since companies would scream "Poor pitiful us" and fines would be the cost of doing business.

Italy had forced KYC for all mobile numbers at least since the early 2000's and no, it doesn't fix the spam/scam calls problem at all.

No.