You're incorrect in your latest email; Etihad do not provide STPC to all travellers with a connection time of 8 hours, and they did not say that in their first message quoted above:

"However, this is usually only offered if there is no better connection available on the day you wish to travel."

There most certainly are connections available for BER-BKK that do not require 15 hour stops in AUH (see below); if you booked such a connection, then it would have been cheaper than the shorter journeys in order to factor in the "inconvenience" of a long layover. With these savings, you can pay for a hotel, if you so wish.

You cannot save money by selecting a cheaper itinerary that involves an overnight stop, and then expect the airline to pay to put you up overnight.

STPC is offered in limited cases only, and only when there genuinely is no alternative than having an overnight stay in AUH.