I think referring to people by their last name w/o a title like you describe is the "proper/formal" way to do things. It's certainly how I see people referred to in traditional/conservative forms of publishing (for instance, non fiction business books unless the author really knew them on a personal basis)

I think it's the difference between eulogy and obituary; one is designed to arouse emotion, the other to inform.

The question is: does the first vs. last name convention tie to the speaker (typically eulogy will be written by one who knew them, obituary likely not) or to the listeners. I'd suggest the latter, the same way a priest who didn't know the dead person would give the eulogy as if he knew them, because he is talking to their friends and family.

If 30 years after your death I were to write about your work then I would probably use full name or last name, whereas if someone asked "was he friendly on HN" then forename might be more appropriate?

I agree that using someone's first name can be overly personal and insincere. OTOH last name comes across as formal and stuffy.

How about using their internet handle?

That depends on whether it's a personal handle or a public alias. eg an IRC nick on a private channel would be no different to their first name or a real life nickname amongst their friends. However a public handle like dmr (in the case of Dennis Richie) is -in my opinion at least- little different to how recording artists and actors often choose a pseudonym (eg David Robert Jones uses the stage name David Bowie)