That alternate M is really cool.
Might just be me, but the overshoots on C and G look too big. X, Y and Z feel a bit weedy in comparison to the others.

I mean I think they are extending slightly too far above the cap height line and below the baseline. As for X Y and Z, I don't think they're bold enough and also not wide enough.

И: In isolation it looks unstressed, but in context you can see that the verticals are lighter than most verticals and the diagonal is heavier than most horizontals, which indicates the normal stress of a Latin N. For И, start with an H. With regard to the weight of the verticals and how far apart they are, you want it to be as similar to H as is reasonable.

Thanks to both of you!

I'd make the counters in B bigger. They don't have to match E, they just have to look right. Look at Antique Olive Nord.
I think the white space in A and V is poking slightly too far towards the top/bottom (respectively) of the glyphs (1). I would either (2) bring the stems further into the centre to push the top of the triangle in A down (and the bottom of the triangle in V up). Also might need to lower the crossbar in A slightly to accomodate, or (3) trim the counter with a kind of inverse ink-trap, but this might make them look a bit too 'techno'

Okay, I have just updated with a PDF sample of my work thus far. (Please forgive the poor spacing and lack of kerning!) It includes all the uppercase Latin and Greek as well as all but three of the basic Cyrillic capitals. At this point, I am already aware of a couple of issues. I have difficulty giving Xi enough weight and maintaining balance between the top/bottom strokes and the middle one. (Any advice?) I'm also feeling as though Theta and Phi (also Cyrillic Ef) aren't quite right. I also think that the whole Cyrillic alphabet has some width inconsistencies to it, but I can't tell which particular glyphs are producing this problem.

Also, I've included a couple of alternates for Cyrillic (De and El) similar to the forms seen in, for example, Paratype's Quartal and Mellnik Text or 4th February's Neultica 4F. My concern, though, is that these alternates might clash with the (purely stylistic) alternate Latin A. Would it be sufficient to simply separate these alternates using OpenType language-specific features, or is it better to omit the alternate A entirely?

Good point. I'll plan on leaving it out of the production version unless I decide to add more alternates to complement it.

The X is still, I dare to say, plainly wrong. The two bars must appear as being crossed over.

I don't think that's true. Most fonts have some optical compensation at least, and many (especially black and bold styles) don't even make a pretence at crossing. Impact and Antique Olive:

Optical compensation, yes of course. But measured.

Having experimented with some different shapes, I've come to agree (at least somewhat) with Mr. Stötzner: the join on the original was too wide. The new incarnation makes a fair compromise, I think, between width and optical "correctness". I'm considering adding a little more weight, though.

Yeah, I think exactly in-between the original X and this one would be spot on

Not too-optical but too-top wide or bottom too-narrow. Well the whole letter seems a bit too narrow comared to others anyways.
And please take it to your heart what @hrant said about /N/ and /X/. Have a disjoint /X/ is a much better choice in your font than crossing it. That just doesn't work in your case.

Here we go again: I've thickened it up a bit and shifted its weight downward a bit to relieve its top-heaviness. I also updated the picture at the top with 0-9 as well as minor adjustments to some other glyphs.

Compared with that one, I could almost call my X "mildly" disjointed ... and it makes me glad I don't have to deal with slab serifs here!

Do you mean don't lose any more (i.e. latest version), or it was better in the previous (more top-heavy) version?

I'm still undecided on that. I'm keeping it the way it is for now, but I have a copy of the old X too. My opinion may well change.

I'm approaching a difficult decision: all caps, small caps, lowercase, or unicase? I think that lowercase might prove difficult if not impossible (particularly e, s, z, and a if I do the "two-storey" kind). Small caps, depending on how small, could pose the same problem. Unicase appeals to me, although I have questions there as well. And going with all caps seems too easy. Recommendations?

I'd go for lowercase with massive x height. It'll be a challenge but not impossible, and typefaces with lowercase are so much more usable than those without. Even for display.

I like the chunky-ness of this font. The disjoint style fits perfectly to this font.

I personally would open the /C/ and /G/ a bit. The top and bottom curves press down hard on the squarish forms, closing the inner space. Lifting those endings could bring the form closer to the /S/, which is very nicely done.