Engineered wood will always be the most stable. In order of preference, MDF, Particle board, OSB, Plywood, Dimensional lumber. (in terms of stability for this specific purpose).
No idea, not a concern of mine. All the parts are minimal so it can not get more efficient. If it is too much use bigger spaces between struts.
Connect two pieces. If you want to use metal or dimensional lumber it is not a big deal, connecting two pieces of wood is usually not a huge deal either. I gave you a tip, it is not a hard fast rule, I do not recommend dimensional lumber due to stability. If I remember right, you were recommending it, I just gave you a suggestion due to density and stability. I also do not recommend metal only because it is expensive. If you want to build this out of Aluminum box tube that is perfect…but beyond the philosophy of my designs.
I do everything in my power to design things for everyone in the world to be able to replicate them as inexpensive and as easily as possible for the best possible results. Every thing I design can be better with more money, better tools, exclusive or specific material or parts. I try not to lock people in.