i do understand your points tho i must say i,m in no way qualified in structural strength of chassis designs of any vehicle, as Fabby has suggested, a full roll cage is the "better/safer option" if the car builder wants total piece of mind etc, i was just using my own personal experience of "head safety" to be able to offer ( i hope ) a credible personal point of view.
As you say the Roadster is "to build a sports car on a budget" but doesn,t give a budget figure & so the safety aspect is one area i believe is worthy of spending more rather than less if the individual builder wishes/can afford to.
on a lighter note i prefer the middle diagram of roll bar design
again said in a polite & hopefully constructive discussion manner.
cheers
andy