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.

There are several problems here as I see it. First and foremost being the fact that someone let these out of the shop like this. That right there is very concerning. As LWO stated this attempt at a "Henerybuilt " look was done by a shop that is not capable of that level of work. I've done this kitchen a few times. You have to be well versed in all aspects of the trade to pull this off well. Mostly you have to care even if it cost you. As far as the defects most of what I see is glue over spray that was not properly cleaned off then showed up when a clear coat was applied. This banding was not done with a machine but by hand with contact cement spray. An edge bander doesn't leave that kind of glue behind or it would be pointless. This glue needed to be cleaned off with a solvent and sanded carefully. These are common problems with low-tech equipment and unskilled labor. There could also be some burn through of the veneer where it meets the banding which is where that is going to happen when you are trying to level out the face especially if what I suspect what was used was off the shelf pre laid-up walnut plywood veneer which is really thin. This also has to do with the dent and ding issue. The thin veneer but mostly the substrate it's bonded to. The way a company like Henerybuilt and the way I also do it is to use selected architectural veneer in raw bundle form and have it laid-up yourself. The quality and thickness of the veneer is much higher and you can request any substrate you want like Medex which is what I like to use. I wouldn't ever use the 1/8" wood banding (either does Henerybuilt) don't like to see a seam around the door. Just use the thicker edge banding 1.2mm and blend it in. Sorry none of this what should have been done helps you right now but the more you know the better you can make your case. The main issue is the glue and the clear coat. Never heard of catalyzed enamel unless you mean catalyzed varnish. Don't have a lot of experience in extreme temperatures (it's 80 today here in LA) but I know there can be all kinds of curing problems and the materials have a recommended range of use. Even here many people use equipment to heat the material and temp controlled rooms for a better finish. As far as a fix without knowing exactly what was done it's hard to say if anything can be done. Stripping off the finish and cleaning the glue if veneer isn't damaged then re-coating is a long shot. Making new doors is probably the way to go. I would just address the issue of the discoloration with them and see what they have to say about that and how they plan to correct it before I got into telling them how to do their job correctly. Let us know.

If it's real walnut, it should get nice and orangey pretty fast. How old is the cabinetmaker's sample that is insufficiently changed? It shouldn't take long, but if he keeps it in a dark box or something it won't change properly.

Walnut can certainly show a good deal of variance in grain structure and color/tone. Mahogany is notoriously bad for this as well. A lot of times, when a cabinet shop purchases material, that solid wood may not be coming from the same trees or forest as the veneers are. This is where "Standard", "Select", and "Premium" grades usually come in. There needs to be a selection process prior to assembly to ensure that the grain structure or colors are going to match in the end. This slows down the process, ergo + labor, + lost opportunity cost in the shop -BUT- a more uniform product in the end. With veneers, you may be paying extra for grain matching, book matching, or slip matching—which can get very expensive, very fast. All of these things drastically increase the amount of attention needed in manufacturing and it caters the entire assembly/milling process to whatever the end-goal is. As for finding or making frame-and-panel doors that are entirely veneered... We can't speak to that since it's something we don't do. The only benefits I can imagine this has is driving price down, speeding up production, and allowing greater control for a uniform color/tone. All of this at the cost of a product that looks real, but... isn't. EDIT: I realize I never answered the last part of that question. If a person has the mindset that when a door gets scratched or dented then it needs to get replaced then a veneer door will last just as long as a solid wood door, with the exception being water damage. More specifically, what I'm talking about here is a door's ability to remain looking presentable until it needs to be addressed. In this case, veneered doors and solid doors will last a similar amount of time. When talking about durability, a door's ability to "hold up over time", IF you have the mindset to fix or refinish a door, is where a solid wood door will shine. Because the veneer on a veneered panel is so thin (industry standard in the US is about 0.6mm, premium veneers can get thicker), we're talkin' less than a 1/32", it becomes incredibly difficult to sand through a finish and stop at the veneer in a repeatable, time-efficient manner. With solid wood, you can work a little bit past your finish and sand into the surface of the wood and there will still be more wood... because it's 100% solid wood! Solid wood doors, if they're made from a nice species of wood will likely be worth the time and effort to refinish, thereby increasing their lifespan. A decent finish should last at least 8-10 years. A great finish will last 15+ years, depending how they're handled. Keep in mind, completely refinishing doors isn't cheap. Many times, a good finish can be 75%-100% the total cost of the unfinished doors themselves. If cabinet doors get refinished professionally, you will get charged for the normal cost of a good quality finish, PLUS the time/labor of sanding through the old finish and making repairs. All of that will cost maybe 75% the total value of the original doors (including their original finish) but should cost less than buying new solid wood doors. If you're a DIY-er or you "know a guy", many times you can get this done cheaper. Be warned though, good finishing requires a lot of experience, talent, and quality products.