I think the track plan looks good to me. I think the link between levels at 3.3% would be fine. The 4.8% might be a little steep, but I will let others with more experience comment on that one as I have not had much experience with grades. That is one thing I need to add to my layout somehow is another level, I think that adds a lot.
I do think some spurs or sidings would add interest to the layout. Even if you do not do any operations I think a few spurs with a car or two on them might make things look more interesting. With my own layouts I usually tend to over-do-it with track though, and end up with little room for anything else which is not a good thing to do. Sampling the different track is good too and possibly the best way to decide which one you prefer. Nothing like first hand experience. I like the idea of double sided carpet tape for trial of the track and roadbed, may be fine for final installation too?
I don't see anything wrong with the overhang if you use 3/4" plywood. I would think that would handle a few inches of overhang with no problem. The 3/4" plywood will strengthen the whole layout when added, but I am wondering if the upper 66" section and the lower 46" section might need a couple of added legs? It may be fine with the top on, but from the picture it looks like those could possibly be weak points? Maybe where the center modules join the end modules? It could be just fine as it is, especially if no one will ever be climbing on it?
I'm no expert here, but that is about all I can think of? I may have even thought of a thing or two that didn't need to be thought about? 
