What is the difference between Finnish Birch, Baltic Birch, and Appleply plywood?

They are all cabinet-quality plywoods used by various manufacturers to make dobsonian telescopes. The look of Finnish Birch and Baltic Birch will reflect the wood used in its construction - Birch. Appleply has little to do with apples. The outer skin is Maple.

What are the differences between standard aluminum, enhanced aluminum, and Beral coatings?

In my opinion, the difference between 89%, 91%, and 96% isn't noticeable visually on a single mirror. I don't think anyone could tell the difference in side-by-side comparisons. A 96% coating will produce an image that is only about 8% brighter than with a 89% coating. However, the difference between having 96% vs. 89% on both primary and secondary can be appreciable since the reflectivities multiply. A telescope having both mirrors with 96% reflectivity would be 16% brighter than a scope having both mirrors with 89% reflectivity. That's the difference between a 11.5" mirror and a 12.5" mirror. I always go for an enhanced coating on the secondary.

However, I don't recommend the enhanced coatings because of higher reflectivity alone. Beral coatings can be tougher than aluminum coatings. Their only drawback is that they are somewhat sensitive to salt environments. I would not recommend them in coastal regions where one can encounter salt fog. Over time, they would degrade faster. The reflectivity increase over a standard aluminum coating is a minor improvement.

There is currently some debate over the durability of enhanced aluminum coatings.  Standard aluminum coatings are generally overcoated with SiO2 (quartz), and enhanced aluminum coatings are overcoated with a combination of SiO2 and TiO2. If properly deposited, these oxides are harder than aluminum and help to protect the surface. However, in some coating processes, the oxides are porous and are more fragile. Aluminum overcoated with SiO2 (the "standard" aluminum coating), generally has reflectivity between 89 to 92%, depending on wavelength.

Is a telescope with three struts as stiff as a conventional eight-strut truss?

A properly designed and implemented conventional eight-strut truss should be much stiffer than three parallel tubes. So, no, three parallel tubes aren't necessarily as stiff as the conventional truss. On the other hand, three beefy parallel struts can easily be stiffer than an eight-strut truss built with tubes that are too thin. I've even looked through dobs with eight properly-sized tubes and have been disappointed with their performance. There are a lot of factors that influence how a scope performs. Its an issue of what's required and meeting all those requirements. I have done a lot of calculations and have built a number of scopes to develop the Mark III. The parallel tubes exceed the requirements. I have also paid a lot of attention to other design factors. In the final analysis the issue is whether it works, not whether its as stiff as a particular truss.

Other telescopes I've seen use a shroud. Why don't you use one?

In dark skies, the shroud isn't necessary. The contrast is great. The most important feature is the baffle opposite the focuser that only allows light reflected from the diagonal to enter the eyepiece. However, projected light on the inside surface of the baffle (from the Moon, for example), will noticeable reduce contrast. A wraparound baffle or a shroud will improve contrast in this situation.

How long does it take to assemble and disassemble?

Starting with the scope packed up in my car, I've timed myself at eight minutes. However, 10-15 minutes is more typical of first time users and careful disassembly in the dark. Although collimation upon reassembly is close, I always take a little extra time to tweak the collimation for optimum performance.