Buckling is from expansion, which is caused by increased humidity (typically for several days to weeks) compared to when the glue was applied. It seems unlikely that a fireplace would be the culprit, more likely a seasonal change from summer to winter. But you normally expect a substrate such as MDF to be very stable regardless of humidity swings. The whole thing is kind of curious and I can understand why your cabinet maker is scratching his head over it. Anyway, to answer jerzeegirl's question - veneer doesn't curve or bend easily. It is possible to get it to curve but usually a lot of care and special handling is required, such as steam or other wetting agent, then a vacuum box or special jig for keeping pressure applied while the glue cures. You normally wouldn't veneer a sharp 90 degree bend with a single continuous piece of veneer. A better solution would be two pieces applied to each face, or better yet some form of solid edge-banding depending on the application.

I think the grain direction is just a personal choice. Go with what you prefer, not what others tell you they like... Having said that, once you talk to your cabinet maker, you may find that there is a huge price difference if the grain always has to go one way. Sheets of plywood are 4' X 8' (unless cabinet maker has access to bigger sheets/rolls of veneer which he makes his own plywood with. The teak for our bathroom vanity is $300 a sheet. For that reason, I am having the cabinet doors over the toilet going with the grain opposite the vanity---just because I don't want to have to pay for another $300 sheet of wood. I'm also having the vanity 24" deep rather than 24.5" deep, again saves another sheet of plywood!

I went with my cabinet maker to select the veneers too and I found it quite helpful and enjoyable. He had pre-selected what he thought were good high quality veneers and then he showed them to me to see my reaction on grain patterns (heavy, subtle, etc). It really made me confident about my choice. As GreenDesigns says, it's really a matter of selecting your personal style.

General Finish. Arm-R-Seal. Wipe-on. Oil urethane Data Sheet You could also use 100% Tung oil with proper drying time between coats. 100% Tung Oil