There are always two design spirals you can use. Make it lighter so the loads are less, so you need smaller engines, so the boats is lighter, so you can reduce the scantlings, so making it lighter etc
Or you can make it heavy, which needs bigger engines, thus increased load and so scantlings have to go up thus increasing weight
Successful multihulls tend to follow the first spiral. Go a bit further north than Alaska and you'll find the local boats are all made from seal skin.
I have done the Juneau/BC trip a couple of times. There are places where you can dry out. Although one time in Tracey Arm we were touching a cliff and our echo sounder read 900ft!
But I'm not sure why you would want to beach your boat. Indeed I haven't seen any cruising motor boat doing that.
I would suggest aiming at 6T, say 12000lbs, and try to reduce your enthusiasm for big bits of oak! (the photos you posted of your wood work look fantastic BTW)
I'd also tend to go deep rather than wide. For efficiency aim for a 10:1 hull length/beam ratio at worst
Don't even consider the one central prop idea on this size boat. One central engine with twin hydraulics is an option though
You won't ever capsize a powercat. Don't worry about it
I looked through my saved data of other powercats. This one looks a bit like what you are after http://www.chernomor.su/shop_chernom...ran-fr-01.html
(Note: google translate does help!)
Richard Woods of Woods Designs www.sailingcatamarans.com