It is possible to install the underground app but [...] error 500
Same here, still not working properly: untouched / unrooted
4.6.6.1 (tested on GSM unlocked and DE variant):
Updating the app is possible but when it's accessing the Underground apps:
Titanium can't help in this situation, no matter if you restore an app that was downloaded via Underground or even the Underground app itself, since the "device" will be identified each time...
Since it works on custom ROMs, things like IMEI, WiFi MAC address, make no difference in the process but if you look at the permissions of the Underground app, it's allowed to gather lots of information about the device, more specific: a lot of them refer to features only found on Fire OS devices, so maybe it helps to blindfold it to a certain degree... cue: trial and error. ^^
small update:
With editing only the 'build.prop' to a large extent (including removing Fire OS and Lab126 exclusive entries), I got to the point where it said "The specified device could not be authenticated. Please try again".
Combined with the fact that I had almost all Amazon related apps deactivated (not deleted) at his point, it still wasn't able to receive the Underground apps page inside the app...