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If you follow their advice it's actually pretty easy; what they suggest in the article is just disabling the 'custom' error functionality for those php functions so it doesn't present 404 any more. This means copying and pasting those 5-6 lines into one file inside your root web directory. They end the article with another code snipped that will actually log those errors into a log file on your system instead of letting them go 'invisibly' (because once you disable the custom error how do you know if it's happening any more?).

SEO wise it's hard to say. Obviously in an absolute sense less errors = more better, but the 404s should be considered in context of your overall site's performance. What you might want to examine is why did your error rate apparently triple around February?

This depends on what you're doing and how. Most European countries have a 'nameplate' requirement (at least minimum contact info) for sites that operate as businesses. There is no such formal requirement in the United States that I'm aware of. There is however a perception issue, especially if money is changing hands, that you generally should have a public WHOIS.