All points correct except c). Egress filtering happens on the ISP side, there's nothing the DNS server can do once it gets a request with a spoofed source IP.

But since operators of authoritative name servers are more likely to be sophisticated they could notice an ongoing attack and throttle down the replies without negatively affect anything else. In fact, that protection could be built into the server code. Simply throttle consecutive replies to the same requester to a sane amount. There is no legitimate use-case where the same person would make a humongous amount of consecutive requests from an authoritative server as responses are usually cached. If that's done, an attacker wouldn't be able to coerce authoritative servers into flooding a target, they would just send replies at a slow rate(after an initial speedy response) and no significant amplification would occur.

As you state in d) there are a lot of open recursive servers out there that are unlikely to be updated or managed by someone sophisticated enough to respond to attacks like this. Whereas this is less likely with authoritative servers.