John,
Would a glue-lam or even a steel beam on either side be strong enough to eliminate posts B/E? Steel posts with a welded saddle on top would provide added strength and collision protection from your 911
I also imagine maintenance with steel better than wood around ground level. Powder coating, at least the posts, provides very long term protection.
Potential snow loads might be a bit much for thin, corrugated plastic laid flat. Thus your idea for add'l support is a good idea. A pitched roof can shed more snow, but it also changes the design. A slightly pitched rock or composition roof with a low-profile skylight could provide the weatherproofing you want along with some illumination underneath.
This design would look right at home in a So. Cal. environment. I'm looking into building something like this in the future.
Good luck,
Sherwood