Well, I was all set to sit back and watch your very interesting project, but decided I would add my 2 cents. The lovely curved elements you want to use make a pretty complicated structure from a "what will it do, how safe will it be and what will be likely to fail" perspective. I realize you are not building a road race bike, but I am getting the vibe that you do want it to be functional and not dangerous. I say go all in on the curves and go forward not expecting to put a million miles on it. With that in mind, I offer the following suggestion. It will still be bendy, but "eagle eye" engineering says it should have pretty good lifespan before it fatigues noticeably. Even bendy things need loads spread out a bit to not have them gang up too much on a particular point. I will leave it to you to conjure gusseting if you like this direction. Looking forward to see where you go with this project!
