It feels like the perfect long-distance bicycle touring device has not been offered for sale yet. I'll try to answer a couple of the questions I posed above (thanks to ACA mapping folks for having a very general discussion with me).
"It sounds like the eTrex 30 would be superior for downloading waypoints. How many waypoints are in an average ACA map section? It sounds like the Edge Touring might be simpler from the standpoint that the maps are pre-loaded. How many waypoints will it hold for when I get to the ACA portions of my ride?"
As a very rough approximation, it sounds like an ACA section will normally have over 200 waypoints for turn-by-turn navigation alone, and additional waypoints for points of interest. A single section might also have well over 1000 waypoints. The number of waypoints per section will depend greatly on the number of turns and the number of services available. More remote sections will have fewer waypoints. For example, it appears that TA section 1 has about 250 waypoints for turn-by-turn navigation alone.
The Touring and Touring Plus and Edge 1000 are all limited to 200 waypoints. The eTrex 20 and 30 each hold 2000 waypoints. So based on this information, I would expect an eTrex 20 or 30 to hold between 1-2 sections of ACA maps with points of interest, and maybe as many as 4 with just turn-by-turn navigation. However, it does not appear to me that the eTrex 30 is able to use OSM, and it does not come pre-loaded with cycling suitable maps. This will create an additional cost. The value of OSM for long-distance touring may be limited at this point (OSM relies upon user input, so more remote areas may be less well mapped and updated), but it provides a degree of flexibility, and its free.