1) Wood doesn't expand or contract to any appreciable degree due to temp variations. The expansion, contraction, and general movement of wood is due to variations in moisture content nearly exclusively. Now, I can tell you that the humidity levels in Saskatchewan range between very dry in summer, to extremely dry in winter. I exaggerate only slightly. Apart from the initial drying of the wood, there won't be a great deal of variation here.
2) I am not sure at this point where people are expecting cracks to develop in this thing. Front, back, top, and sides are to be made from single widths the widest stock readily available, i.e., 1x12 (which is actually 3/4" x 10 7/8", but that's a different story). To the extent that they are going to change in size, they will tend to do so in unison (changes are almost exclusively in width - length doesn't change hardly at all), which won't put any significant strain on any individual piece. Crack-causing stress is most likely to occur where one has a longish chunk of wood with the grain running lengthwise firmly anchored to a panel or some such where the grain is running perpendicular to the longish chunk. In such cases, moisture level variation can result in the panel wanting to contract in width, but the lengthwise piece won't want to contract with it. Since the ends of the panel are fixed in place, something's got to give, and you end up with cracks somewhere in the middle. My desk is actually potentially susceptible to this, since I didn't do much to account for it. We're coming up on two years for it, and nary a sign of cracks yet, but it's possible that in the midst of a winter after a relatively humid summer something might happen in the future. Anyways, there are no such construction points in this design. There were in the initial drawing, with the right panel fixed, but in the current design, the only place where there's a joint with the grain running perpendicular is where the mobo backing is joined to the top and bottom. And if that board develops a crack, it just doesn't matter, as it's hidden from view.
I am reluctant to build an inner case from mdf or whatever and face it with pine because I don't know what I'd be gaining. I guess I could build a box from 1/2" mdf with open sides, and then face the top and front with pine and add the side doors (making doors with a pine panel fixed atop mdf is a good recipe for developing cracks in the pine, unless we're talking about a sheet of veneer (blech) or having the pine floating loosely atop the mdf (structurally no different than pine alone)), but to what end? It wouldn't be stronger - it would actually be weaker, as screws don't get nearly the bite in mdf that they do in real wood. It wouldn't be any less resonant, as mdf is prone to resonating as well. It would, I suppose, be a tiny bit less prone to cracking, assuming that I anchored the face pieces in such a way that they could float relative to the backing, but I just don't see where crack-inducing stresses are likely to build up on the top and front of my design to begin with.
3) Re: snug-fitting doors. This is an issue, but I think I have it under control. I'll be running a cleat along the inside of the top and bottom, so that there's 3/4" backing inside the door all the way around. This will be covered in some sort of thickish fabric. The right door will be closed by means of two posts passing through the framework at the front and anchored with wingnuts. (Note to self: draw that frame piece in so it doesn't get left off the cutting list.) In ordinary operation, it wouldn't be opened, and should be pretty solid. In combination with the mobo backing board, it should be plenty to keep the case as a whole solid. The left door will be held shut with some sort of cabinet hardware, likely the sort where the arrowhead-shaped thing on the door pushes between two springloaded rollers. I'll be looking to get a rather substantial set of such items. Snug fit over time should also not be too much of a problem. Size changes vertically will be negligible, as the grain will be vertical. The greatest expansion/contraction effects on the case, though, will be seen in the width of these doors. However, it should be possible to adjust the position at the hinges slightly if they become overly tight or loose.
I'm hoping to get the cutting list drawn up today yet, and pick up the lumber sometime this week so it can sit around and acclimate to the humidity of my apt. Then we'll just have to wait for work to be slow enough that I can take a few days off and head out to the farm (so that I can use all of Dad's power tools