Can't believe I missed this thread until now. Great stuff! Particularly the caps.
In the LC, I feel like the /o should be wider (it looks vertically elongated right now), and some letters feel very preliminary to me (g; v–z; R). The /n/m stay away from the all-important midline a bit too long for my taste—perhaps make them narrower or allow them to dip through the midline a bit?
Have you tried a narrower solution for /a/b/g/d/p/q? I like the intermittent wideness of the LC, but maybe that would be better achieved with longer connecting strokes between letters.
As a longer-term goal, you might want to consider ligatures and contextual alternates to break up combinations that stay above or below the midline for too long.
As someone who usually doesn't much appreciate grots, I find this refreshingly different and rather charming. The tight spacing works very nicely.
The hyphen and dieresis are really quite riqué... but perhaps you could get them to work with just a bit of tweaking. I would suggest making the strokes of the hyphen a bit thinner and giving it some more horizontal space. Currently, the words just meld together, making them hard to read.
I agree /a and /s need the most polish in terms of curve quality, and I would add the tail of /g to that list. I also find the almost-smooth transition of bowl to stem in /g a bit jarring — I would either increase the joining angle (as in /b, /d, etc.) or smooth it down completely.
I'm a big fan of the long /ſ, but not so much of this implementation. The top feels clipped, and I regard the spur as a serif rather than as a letter component, so I would leave it away. In my opinion, almost all sans typefaces are better served with a spurless /ſ, and I even prefer it in many serif typefaces.
BTW, there's no need to «remove» the /ſ from the «well-behaved» version of your typeface. It has its dedicated Unicode slot.
BTW, if you're feeling adventurous, you could even add a few more blackletter-style quirks, such as a /P with a full-height body and a descender, /H and /K with overhead arches, lowercase-style /N and /M, etc... maybe as a third export font, so as not to weigh down your default font.
It can't be that bad if the Duden as been using it on its cover...
The only reason I can think of not to use the cap ß is if it's badly designed (e.g., not instantly readable or disharmonious with the other letters), which is unfortunately rather often the case. Luckily, this is a problem that type designers can freely address by themselves.
As for «predictability»: I would argue that switching fonts will generally change the appearance of a given text more drastically than exchanging SS with ẞ... and while using SS for ẞ can cause loss of information, sticking to ẞ never does. After all, if the user typed ß before capitalization, they meant ß rather than ss. Anyone who can read ß can also read ẞ without hesitation — if not, the ẞ is badly designed.