It seems my problem has been a temporary one.
Some weeks after posting my original question, the settings just started working. I did not actually do anything. I guess "stretch" was not really ready to be supported then under Raspbian, but now it is.
Currently, I have only the following effective lines in my sources.list, and they work fine now:
$ grep ^[[:alnum:]] /etc/apt/sources.list deb http://mirrordirector.raspbian.org/raspbian/ stretch main contrib non-free rpi firmware deb http://archive.raspberrypi.org/debian/ stretch main ui
Lession learned from this incident: Never trust distro maintanersmaintainers to reliably change generic suite names like "stable" or "testing" in a consistent way. They may change the name in on repo, but not (yet) in the others you might be using as well.
Better use explicit suite names like "stretch", and change them manually when the next dist-upgrade has arrivesarrived. This can save you a lot of headaches during the transition period between the dist upgrades.