I have run into this same problem, but in a different sort of manner.
a week ago i had installed a functional Minecraft entirely within a portable USB drive. With two lines in a batch file
set APPDATA=%CD%\Data
java -jar Launcher/Minecraft.jar
((previously i had tried it with the old "alternative" .exe that ends in the same results))
needless to say i have deleted, reinstalled, changed installers... and yet the result always concludes in the launcher getting stuck "unpacking"
Before i decided last night to reformat my thumb drive everything worked fine. Now it is reformatted exFAT and this problem arises. Unsure if there is causation.
the alternate launcher is the only way to begin my process in the first place, and yes that is where i started, it results in the same hangup as running the .jar file version.