(1) about the difference in meaning between 「自分について書きたいことがあります。」and 「自分自身について書きたいことがあります。」.
According to the dictionaries 大辞泉 and 広辞苑 「自分自身」 emphasizes 「自分」.
Martin (p. 1050) translates both watakushi jishin and watakushi jibun as "I myself" and
translates watakushi jibun jishin as "I myself (in person)..."; "I my very own self" - which are ways to emphasize "myself" without the use of italics (but it would depend on the context if a phrase like "in person" can be used in an actual translation).
None of these sources write anything about shifting emphasis elsewhere in a sentence that might contain jibun jishin (and surely Martin would have mentioned that if it were the case).
(2) a translation of 「自分について書きたいことがあります。」 would look something like "There are things about myself (that) I want to write about.". The variant 「自分自身について書きたいことがあります。」 puts emphasis on "myself" therefore in a translation one could use italics, or use a different phrase, or leave out the emphasis, all depending on the context. For instance: "There are things about myself I want to write about." "There are things about my own person I want to write about.".
自分自身 in Yahoo! JAPAN-Yahoo!辞書 / 大辞泉
Samual E. Martin (1987 ) A reference grammar of Japanese