No roof planned, just deck & railing. Snow loads calculated based on a worst case (flat, exposed roof) scenario. A sloped roof would perhaps double dead load (to 20 lb/sq. ft), but it might significantly lower the snow load, for no net change (but would need to check, if it ever comes to that). By the way, the snow load is almost half of my total calculated load.

Decking + tension cable sounds like a good plan (even though the wood won't last 500 years!). I don't want this to fall down.

Gave some thought today on how to assemble this thing...a bit tricky. I think two temporary guy ropes/wires from ~3 feet out on each beam back to the tree, ~3' above the planned attachment point and off to either side, will let the beam dangle in place, allowing "easy" adjustment of horizontal (level) and lateral (perpendicular to tree) positioning. I can then mark, drill, and bolt in the beam support block, fit the strut, mark/drill/bolt it's block, etc. Also plan to tack on a temporary 2x4 between the beams to give lateral stability, prior to proper decking installation.

Fun project, but it is taking some serious time & work! Thanks for all the great ideas and suggestions.