1) Where can I read the original story of Brahman splitting itself to create the universe (but an English translation)? Which Upanishad deals with the story aspect?
The story or description of creation is dealt with in some Upanishads, but I will tell to you what the Taittiriya Upanishad says, since it explains the process of creation in more detail than other Upanishads.
From the English translation on sacred-texts.com:
2.1 - From that Self (Brahman) sprang ether (âkâsa, that through which we hear); from ether air (that through which we hear and feel); from air fire (that through which we hear, feel, and see); from fire water (that through which we hear, feel, see, and taste); from water earth (that through which we hear, feel, see, taste, and smell). From earth herbs, from herbs food, from food seed, from seed man. Man thus consists of the essence of food.
2.6 - He [Brahman] wished, may I be many, may I grow forth. He brooded over himself like a man performing penance. After he had thus brooded, he sent forth or created all, whatever there is. Having sent forth, he entered into it. Having entered it, he became sat (what is manifest) and tyat (what is not manifest), defined and undefined, supported and not supported, endowed with knowledge and without knowledge (as stones), real and unreal. The Sattya (true) became all this whatsoever, and therefore the wise call it (the Brahman) Sat-tya (the true).
2.7 - 'In the beginning this [Brahman] was non-existent (not yet defined by form and name). From it was born what exists. That made itself its Self, therefore it is called the Self-made.
What these verses mean is that Brahman transformed a part of himself into this universe. That is creation.