I think it would be good to remember that the boat you have begun building is an exotic, high performance racing sled, purposely designed and built for the EC race. The cabin is really not much more than a small cuddy to serve as a small shelter for the crew to get out of the weather to get some rest. Those waters, those conditions and for that crew. Any attempt to deviate much beyond that and modify it to suit different needs is probably going to mess up something somewhere. That should probably became your mantra and something to refer back to anytime you get the notion to deviate. As Joe suggest, stay on the path.
As for the Sage 17, a cabin like that might be better, but I can say with almost 100% certainty that it is being built upside down in a fiberglass mold that was taken off a mocked up plywood version of the same cabin top. It starts with a layer or two or three of gelcoat sprayed into a the mold (after about 20 coats of hand rubbed mold wax has been applied), then mat, then glass and the carbon fiber, then the balsa core, then more glass or carbon to complete the sandwiched layup. It is a production boat and they can justify all the labor and expense to build the molds because then intend to build hundreds of them. I don't think you can duplicate the shape (perfect) or construction method any other way. The bottom or hull half of the boat, with the signature Montgomery lapstrakes, is done the same way, as is the liner pan in the middle. So yes, a cabin like that might be better, but it's not likely that you can afford to duplicate it for a one of build, and as Joe suggests, it might not make that much difference.
As for a place to work, I seem to recall you may have access to or work from a military base. If so, they may have a hobby shop on base where you could build something, or have tools to borrow and use? The normal process for birdsmouth masts requires a table saw to make the two 45 degree cuts. An alternative, if you have one, is to use a router on a router table. There are special bits you can use for those.
BTW, the boat I have been sailing for nearly 12 years or so is a Montgomery 17. It has a little sister, the Montgomery 15. The Sage 17 was based on both of those boats and is seen as a new and improved version of both. For it's type, it too is a small cruising boat with good sailing performance, as are both Montgomerys. Don't know how long you will be in that area, but there are several Monty's around there. Walk the docks and you may see some of them.
Interesting that you should mention the Sage 17. I can't think of two boat designers who are more alike in their desires for personal comfort (or rather lack thereof) and ability to exact best possible speed out of a boat and they manage to do it in different ways. I find it ironic that Jerry has nothing good to say about a cat-ketch rig, preferring fractional 7/8th's sloops over all else, mainly for their ability to point and go to weather. He still uses the low profile, pie shaped centerboards, but does use a high profile, lifting foil for his rudder. He spends a lot of time tweaking and tensioning the rig and claims it makes all the difference in the world. Get it wrong and she is a dog. Get it right and she takes off passing everything in sight......to the high side. Most of Graham's designs are unstayed cat ketch rigs and they work well too. On our typical six mile race, I suspect that in a good breeze, a well sailed Core Sound 17 would lap my Montgomery 17, simply because the Core Sound boats are so fast. Granted, they are not dragging around 500# of ballast, and can plane, but speed is speed. There isn't much moral victory to be had watching that stern disappear over the horizon. But later on, when I finally do get to the anchorage, and the rain that followed the frontal wind we enjoyed earlier in the day starts falling......in buckets....... I'll be more comfortable battened down in my little cabin. The point being there are plusses and minuses to everything. Pick the best possible fit for you (and the conditions you will be sailing in) and go with it.