How to decide what pathway a reaction will follow, elimination, addition to a carbonyl or nucleophilic substitution?
The nucleophilic substitution with another equivalent of the primary bromide will never be favourable as the nucleophilicity of the enolate generated is pretty low (the charge is delocalized!)
Even if E1cb was a possible route, you would end up with an extremely reactive vinyl aldehyde, which would likely participate in a Michael addition with an enolate in the reaction medium. So, isolating the product from this route would be very unlikely, even though I cannot back my claim up immediately.


If you ask me, both the Michael addition and the Aldol reaction should compete, given the nature of the base the problem has used. Had it been something like LDA (or any other directed Aldol conditions), the greater stability of the enolate would probably have precluded E1cb as a possible route.