I think because "He is about to arrive" implies imminence, that he is literally about to turn up, whereas "he will arrive soon" is more relative and dependant on context. It could mean a few minutes or a few hours depending on that context. If the journey is only half an hour, then "he will arrive soon" could mean a few minutes, but if it's a flight from Australia to the UK, then "he will arrive soon" could mean an hours time.
The pronunciation of uj depends on whether or not the U and the J are part of the same syllable (a diphthong) or if they're in different syllables. For example, tuj would be pronounced like it would be in a one-syllable version of "tooey"; however, monujo would be pronounced like "moh-noo-yoh".
I found a pretty helpful guide to pronunciation on Wikipedia - https://en.wikipedia.org/wiki/Esperanto_phonology