Peanut gallery response of no critical import: this looks amazing. Most attempts at geometric constructivist typography start with something similar, but they never turn out terribly well; in particular, designers tend to get upset about the tipsy look of the S.

I wouldn't bend head over heels to worry about readability, as it makes perfect sense that some of the characters would be rendered horribly on a device being pushed near its limits, especially a) if it were imported or b) if it were repurposed. Consider how awkward some 15-segment displays can be.

What matters most of all, I think, is working within the limitations of the mindset of the creators of the hardware; consider, for example, how poor ASCII is for representing most European languages (unless one has overstrike available.) If you wanted to explicitly expand to include Cyrillic support, though, one thing you could do to help with the rendering of Ë would be to put the pseudo-diaresis/umlaut inside of the letter, which was fashionable in some German-speaking countries during the Art Nouveau movement, roughly like this. Another option, sure to horrify our resident crypto-Kremlinologists (do we have any of those?) far more would be a horrible orthographic kludge like <йо>, which would definitely add to the dystopian "language forced to fit inside the machine" feel.