I knew this tread is pretty old but it's on of the first googel listings when u google a5100 astrophotography
I also got one used and discovered the long exposure noise reduction problem afterwards
But now there is a relativly easy solution to deactivate it systemwide (inc. bulb mode)
you need to download the open memories tweak: https://sony-pmca.appspot.com/apps
after that you just have to enable wifi and telnet from OpenMemories-Tweak app in your camera,
connect the camera and PC to same wifi, open telnet connection from PC to camera, and command: bk.efi w 01070108 00
After that it's disabled.
Manuel
(edit: I just opened this accout to say this, because i think its a fun very cheap camera for some astro photos and might be helpful to some people)