As much that I want to help you an answer would require significant amount of effort. First, I would need to model your frame in a 3D CAD software or break out each subframe element into my FEA software. I would need to calculate the area moment of inertia and estimate the material engineering properties. Additionally, I would need to design a welded connection and calculate the strain, strain and deflections.
You're asking for a full engineering and design analysis - 20 hours of effort?
Here's my best suggestion.. If available, look at similar structural frames and understand the loads which they carry. When a suitable example is found - reproduce the structural details.
If you cannot find suitable design example - go big and strong with the pipe selection. Over engineer it and you'll likely be good.
Of course you could also pay somebody (engineer) to design it for you as well.