The ideal person to answer your question would be someone who has (or has had) both brands of housings. I don't qualify -- I've only owned two brands of SLR housings: Ikelite and (now) Seacam. However, as I have no financial interest in Seacam, I can at least offer an unbiased opinion on the Seacam housing for your camera.
First, yes, Seacam offers manual focus for the 100mm macro lens (as well as the Sigma 150mm macro lens) -- I've used both lenses with MF, with no difficulties. I actually find the Seacam MF knob easily accessible (somewhat more so than the zoom knob, which takes a bit more getting used to).
Second, the Seacam Superdome works well with a variety of wide angle lenses, and is great for over/unders. It's a big damn thing, but once you get it to your destination, it's a great dome port.
Third, you mentioned Seacam's special viewfinders. These are great viewfinders with multiple optical elements that magnify the image and have an adjustable diopter to customize the correction for your specific vision.
If there's anything I DON'T like about this housing, it's the weight and lack of balanced buoyancy. It is quite negative to start with, especially with the macro port, a long lens (e.g., 100mm or 150mm), and/or either of the special viewfinders. The big superdome adds a lot of air space, and therefore some buoyancy, but it is still negative (at least if you're using one of the special viewfinders). James W. came up with an ingenious "buoyancy collar," and I followed his directions to make one of my own (although I did it in multiple interchangeable "rings" rather than one big collar). That helps tremendously in overcoming the housing's negative buoyancy, but since it puts the buoyancy all on the port, the housing tends to want to constantly point skyward. I plan to use the same technique to create some sort of "buoyancy base" to connect to the bottom of the housing itself to make the buoyancy more evenly balanced.
Lastly, you asked about the dealer situation. I have found Stephen Frink's operation (U.S. dealer for Seacam) a delight to work with (I should mention that virtually all of my contact has been with Liz). They are honest about anticipated delivery times (and delays when applicable), and bend over backwards to get things to you when needed. OTOH, bear in mind that they don't keep a huge inventory on hand, so it often takes a number of weeks to get components from Austria. That's not a negative reflection on the Frink organization; it's just a fact of life dealing with a specialized product produced in limited quantities overseas. As for dealing with problems, I couldn't be happier with their responsiveness. It doesn't mean you'll get overnight turnaround, but my experience has been that they will do everything they can to make you happy.
As I'm sure you know, everything about Seacam housings and accessories is soberingly expensive (the Superdome and viewfinders EACH cost almost as much as some housings!). But the craftsmanship is top-notch, the design is well-thought-out and extremely functional, and there is little lacking. I chose Seacam because I wanted the best possible housing -- one that would last for many years. Having now used the housing with two different cameras (1D MkII and 1Ds MkII) to shoot over 10,000 photos, I'm totally comfortable that I "got what I paid for." More important, if I had to do it over again, I'd buy Seacam again.
Hope that helps,