My best advice is to stick with WATASHI as long as you can. It my opinion, it is the only one you need, especially in a professional environment. However, here is another that hasn't been mentioned yet, just for kicks. われ - Old-fashioned way of expressing "I" (oneself). src: http://ejje.weblio.jp/content/%E6%88%91

The words (nande, doushite, and naze) may have a similar meanings out-of-context, why; however because they are used different times, it may be advantageous to interpret their meaning case-by-case. Take the case of “~DESU” for example. What you will notice is that not all words can be used without breaking the rules. In particular, the phrase “NANDE-DESU” doesn’t exist, at least that my perspective what I think. Hope that was of some help.

ホアマイ＝オクナ･インテゲリーマで、I have a hunch that it should be commonly called ホアマイ in Japanese. As for its scientific name, use オクナ･インテゲリーマ. =Sources= Wiki page → http://en.wikipedia.org/wiki/Ochna_integerrima ※ホアマイ＝オクナ･インテゲリーマ(Ochna integerrima） オクナ科の熱帯花木＝日本でミッキーマウスの木・英名:Mickey Mouse plantとして出回るものの近縁種） 参考（アオイソラニ～フトモモ科偏愛日記さんブログより）http://futomomo.jugem.jp/?cid=3 (^.^) [http://blogs.yahoo.co.jp/may33124416/51870290.html]

The answer is "case by case." Both are used in a similar fashion. However, depending on the the context, one phrase may be more appropriate than the other.

For writing an introduction you need to work on your 自己紹介-desu. http://www.guidetojapanese.org/learn/complete/chapter1_summary

１週間前に亡くなっていたことがわかりました。　→ It became clear that he passed away a week ago.

Please ignore the above comment. I meant to have it listed as an answer. Thanks you.

Japanese for going to the store to buy groceries. http://www.noodle.org/learn/221407/learn-japanese-grocery-shopping-phrases Good resource for learning everyday conversational Japanese