I don't think the knee walls will be a problem and you can certainly build your frame w/o attaching it to the walls. It will essentially just be a series of table tops. It won't be as stable as attaching to the wall, but should not be a problem. Think of the modular layouts that are brought to train shows that have no wall support.
As far as electronic protection, there are numerous threads on the Forum that have discussed the need for fuses and TV suppressors, which you should be able to access for more information.
Most hobbyists will recommend staying at no more than 30" wide for your layout, but unless everyone is really short or physically challenged, you should be able to go to 36" with minimal effort. Even though the area under the roof wall will largely be unuseable for running trains, I would still build out almost to the edge of the wall because you can put all sorts of small, short items like sheds, picnic tables, abandoned trailers, tents, campgrounds, scenery, etc. in that area. In fact you might be able to go to 40" wide because that area will likely only have to be accessed one time for each item you install and the "workable area" you will have to repeatedly access will still only be 30" - 36" wide.
Just my $ 0.02
