It is needless to mention that all those "precautions" and "solutions" have to be made manually. The client can only tell you if your coins are refused, which gives no insight whatsoever of whether you're being Sybil attacked or not.
This costs the target victim nothing, and they can repeat it as many times as they want.
How does the victim get to do that indefinitely when the coordinator is supposed to ban him after refusing to sign and disrupt the round? What prevents the attacker coordinator to simply mark these inputs as blacklisted, with DoS protection measure as an excuse?