"1. When I build w/MDF and put veneer over it, the butt joints seem to show up under the veneer after severl months. It always looks great when first finished, but over time, where the veneer is over an MDF edge, the edge begins to show through. Thoughts? I normally use contact cement on top of plain MDF."
I dont know how to say this in a nice way but the problem has a lot to do with the contact cement. Contact cement is called glue in a loose term as I see it. It's rubber cement, wont form a rigid glue line, sun light can loosen the bond. So those are the bad things and people have written small novels on why not to use it. So get yourself some veneer glue from www.veneersystems.com or another commercial supplier.
After your MDF box is all put together. The outside should be sanded down with 80 grit to break the surface glaze and level everything. If you have a belt sander that will work very nice with a little practice. The idea is to level the surface smooth. Hold the sander at a 45 degree angle across the panel. move from left to right and shift down 3" with each pass. when you complete the whole panel. Hold the sander at -45degrees and repeat the process. Do this 4 or 6 times with very light pressure. You dont want to remove a lot of material at any one time. Just take down high spots and maintain (or create) surface uniformity. When all done you should have a nice criss cross pattern for the glue line.
Random orbit sanders are no good for doing this. A stoke sander or thickness sander is great and a sharp plane and a sanding block is your low budget option.
"2. I'd like to have solid wood edges on the front baffle so I can round them or bevel them a bit. What is the best method to join solid to the MDF+veneer and create as little seem as possible? I have regular tools, nothing fancy like a jointer. Do have a router and table saw..."
Make your solid wood edges with your table saw and your router. build yourself a router table if you havnt already. Veneer the entire cabinet and have it all but the last stage of sanded and ready to finish. Apply your solid wood edges directly to the veneer.
If you have lots of clamps, that would be the best, most traditional and most time consuming method. You can use a few nails to hold on the wood until the glue dries and then fill them. You use masking tape to hold on the wood until the glue dries. Space the 1" tape with 1" spaces, that will be enough pressure for any PVA.
"3. I've seen more posts lately from people using solid wood in the construction of their speakers. If I ended up with a front baffle of 42"x13.5" would that be too wide to expect a solid wood baffle to work well? I'd be building it up from narrower boards of cherry, dowled together. Would warping over time be an issue? If so, suggestions?"
For solid wood carcass construction there are more tips and tricks then there are cabinet makers. Pre-dress the wood. take it close to thickness but not at finished thickness. let it stand where it will be assembled for 4 weeks. Sticker board and use a small fan for circulation. No funny business like building in a damp basement then put your finished work in a dry house all air condition and dehumidified. It gets ugly. the horror.
After your wood is all acclimated finish machining it to dimension for glue up. Follow the growth ring rules of engagement for edge gluing or face the circular consequences. 13.5" is not so wide for a face on a cabinet that cubed out, meaning its attached on all sides and is not a door hanging and swinging in the air just waiting for nature to throw it a curve. cross ribs wont do you much good because they grow with the panel and in a few years wont be holding it anymore. there are tricks with screws but dont bother. The big key is stable wood. If you really want to do something, finish the inside of the cabinet too.
good luck and happy building