Pronunciation of "undoable"

• your pronunciation of 'doable' looks correct (for AmE)
• the phonemes you have for 'undo' look correct, but the stress pattern, though formally correct may not allow you to draw inferences about changes well. It is more a word with two stressed syllables (a 'spondee')
• adding on affixes is not independent in English. You have to take in to account the whole word to get it right.
• so the correct pronunciation is your first one ʌnˈduːəbl

As an aside, the schwa in English (or GenAmE, I can't speak for others) is always in an unstressed position, and the 'ʌ' is almost a schwa.