Most of the stain companies say I need to use their “cleaner” and “brightener” before staining, even on new wood. Does that sound right? I’m wondering if they’re just trying to sell more product!

I am planning a treehouse for my little ones (7 and 10) and need to send something to the glorious Home Owner's Association for approval. I've attached a rough sketch of what I'm thinking for the layout and would like some advice on a few points. 1. I want to cantilever one (if not two) corners of the treehouse by about 24" from the corner posts. What do I need to consider? (The reason I want to cantilever is because I eventually want to enclose some of the space under the floor for use as a shed.) 2. I am planning on four 4"x4" posts, each 10' in length and buried 36" deep (with an additional 6" of gravel below that) and in concrete. Can I get away with these posts being only 2' deep instead with 6" of gravel below that? That way, I could get away with 8' posts instead of 10' if I go with a 6' height off the ground. I was planning on the beams to be 2x8 PT lumber with the remaining joists, etc. in 2x6. Thoughts? Advice? Thanks!