Turning off new account signups won't reduce low-quality comments, unless in the rare case that HN is being raided. (i.e. during the Adria Richards incident)
Indeed. If the assumption is that the bulk of current accounts are also low-quality, and that (for whatever reason) account age is a good indicator of quality, you would have to follow up the cutoff of new account signups with a mass deletion of accounts (including OP's) until you got to the 'quality' line.