>To be precise, many library authority records do contain birth and death date information.

>Why are the Birth and Death dates for pseudonymous, discrete literary identities, when they are stated, made identical to the birth and death dates of the person using that literary  identity?

Birth dates are added to disambiguate entities with identical names.

>"Mark Twain" was not used by [Samuel Langhorne Clemens] until 1863; to assign the birthdate of the latter to the former is problematic.

It hasn't been a problem in library catalogs, which have followed this convention for decades. What aspect of it do you see as problematic?

> What gender is "George Eliot"?

Do you mean Eliot, George, 1819-1880 ?  Female.

Also, Karen, if you like, I can add your birth year to your authority record. What is it?