The Dalwhinnie approach does seem to make most sense. I think you could work a good itinerary that way, based on the suggestions above and you're own planning and knowledge of your own speed when laden.
You mention that you've not done much in Scotland before. Apologies if I'm teaching the bleedin' obvious, but it might be worth noting that some parts of the route will take longer than they would do on more traditional long distance routes, due to the rough terrain, some of which will be trackless.
The ascent to the CMD from Steall is one such part, and the route is going to be tough going. Depending on how light you travel, it may be flippin' hard with a 4-5 day pack on, unless you are used to this sort of thing. I guess what I mean is that I personally would think hard about carrying a big pack up that way, but I'm getting old and lazy!
And my knees would break going down the Ben's normal route with a pack on
but that's me!