Overall conclusion of mechanistic experiments

If during the quenching reaction unreacted rearranged Li-intermediate is available in the presence of the already formed target tri-haloproduct a series of side products emerges consecutive non-selective transhalogenation reactions.
With this knowledge we could prevent these side reactions when approaching 5-bromo-3-chloro-2-iodothiophene:

In a first step we rearranged the starting compound under formation of the 2-Li-5-bromo-3-chloro-intermediate, which in a next step was slowly added to iodine in THF, thus immediately quenching the Li-intermediate. As in this case no significant contact between formed target compound and Li-intermediate is made, the desired 5-bromo-3-chloro-2-iodothiophene was obtained exclusively with good yield (70%).