Actually, with the 7D, you should be able to have the filter set at its strongest density and compose/focus with Live View...LV will see right through the filter in decent enough light. I often stack my 10-stop with a polarizer for a total of 12 stops...the 5D2 LV has no problem with it...I suspect your 7D shouldn't either. Just another option to consider...
I haven't thought about that. I spent most of my time playing with the 5D1 today and there's no live view on that. If I get out to the waterfalls this week, I'll have to try using live view. When you do this method, does autofocus work, or do you have to use manual?
With my experiments on the 5Dc today, I'd set the ND to 2 stops, lock focus with AF, switch the lens to MF and set the filter to 8 stops, and shoot. Seemed to work well. The only lens I couldn't do that with was my 100-400l at 400mm. I may have not had enough contrast in my subject.....more experiments to come....

