Looking at the design, the corner seams need welding so you might as well weld everything. Its really hard to get that tab & slot construction to line up properly when you are bending a frame like that, making welding even more attractive.

With hot bending, I am only going on what the manufacturer recommends - they just mention that scale may form after heat bending and to remove it; nothing about inert atmospheres. As I understand it, you only need gas if the Ti is molten, which is a much higher temperature. Everyone using titanium should memorise that linked manual - its good stuff!

On Mr Mangle, I bent these 4mm brackets with a vice, an adjustable wrench and a torch you'd get from any hardware store:



It was quick & easy, although better equipment would have given a bit better result.