My problem turned out to mostly likely be the result of switching networks shortly after the first download attempt. It left the phone in release version of 4.6, while it really was still 4.5.
Setting aside the annoyance of the upgrade process not being atomic, I was able to restore the zypp conf file to the 4.5 version and proceed with smooth upgrade via GUI. (I use XA2 dual-sim, by the way.)
I was worried about the Android App Support, thanks to the other reports, but I was positively surprised by it running well on the first try.
I am also very surprised at how much memory management was improved in the Sauna — to the point of regretting buying a 10 V. I am now able to open and use many heavy applications simultaneously, and still get working camera. I am very happy to see this development! Hopefully this fixes the recently discussed issues some people had with the base phone functionality.
At this point the only pain of Early Access I have is the missing Amazfish release for the new system version, which is actually a good thing — in my experience the XA2 release of Sauna is Platinum.
PS: I did not have an opportunity to test voice and text messaging, but I have strong trust in those aspects. I am pretty sure the reports of such a failure would have been made long time ago.