Prototype projects are few and far between, and rarely done by a single FTE under any circumstances, even on light aeroplanes - certainly not on bigger stuff.
I was running small trials about a year after starting at Boscombe Down as the main FTE, growing to quite significant trials after a couple of years.
I have run prototype programmes as an FTE, the first one was probably 6 years after I first started working full time in Flight Test, and that was on a light aeroplane - I doubt I'd have got anything much bigger at that stage in my career.
Series aeroplanes are routinely done by one TP and one FTE in most environments, but again a pretty experienced FTE.
G