Perhaps I can add a couple of observations that would assist the discussion.
First is the presumed role of 'Congress'. In my direct experience in DC, the people pushing the JSF concept were not 'Congress' - is was the Office of the Secretary for Defense, known as OSD. This is essentially the 'Centre' staff for the Pentagon, and they had, and still have, real power in the area of programme decisions and even more importantly, budgeting. In truth, Congress don't have the depth of technical analysis required to reliably develop their own programme assessments - they rely heavily on what they get from OSD via the Oval Office.
The concepts for JAST and JSF were, in large part, developed within and promoted by OSD. And the main driver for this push was a perception that trying to go forward with single service programmes for large, twin engined tactical aircraft (mostly with two seats) was always going to be unaffordable. I heard senior OSD staff refer to the F-22 as a 'failed program', and looking at the original versus actual ISD and costs for the aircraft, I couldn't disagree. (I would add that the F-22 is (in my view) a great design and a fabulous performing aircraft - but unaffordable, even for the USAF).
OSD, USAF and USN technical assessments, based on ASTOVL and many other test programmes, was that engine and airframe technology wasn't capable of producing a viable twin engined, two seat, STOVL airframe. As a result, OSD staffs prepared the briefs that pushed for a single engined, single seat, aircraft, using STOVL as the prime reason for that configuration. All these years on, some posting on this thread will disagree with that assessment. All I can say is that is what I saw happen.
OSD, the USAF and the USN had also studied (and modelled at some length) the relative impacts of airframe and avionics systems on combat efficiency. The USMC had also seen what the RN were doing in the mid 90s with the Sea Harrier FA2 - a badly limited airframe given a massive combat efficiency boost via an excellent radar/missile combination. All of that fed into the early iterations of what eventually became the JSF requirements set. This has led to the sort of aircraft that has emerged from the programme. Again, OSD people were key players in this process.
Second, concurrent engineering. I was at Fort Worth in the early days of the programme, and I didn't detect any huge focus on 'concurrency'. I did pick up that the programme had set itself some incredibly ambitious goals (and dates) for freezing the airframe and systems design. In part, this optimism was based on the depth of the X-35 programme, and the other precursor technology programmes the US had carried out. Finally (and in hindsight mistakenly) there was an assumption that new CAD systems would give LM's design team a much higher 'right first time' figure.
All that unravelled in 2004, when the customer told LM that the aircraft was badly overweight. What followed was a huge structural redesign, but the schedule screws were kept firmly 'on' - my own assessment of what followed( and that's all it is) is that LM were simply not able to get the aircraft to the level of design maturity (stability) in the time allotted. I'd also add that LM had some weaknesses in the slightly arcane areas of configuration control and change management. (I'd further add that they were by no means alone in this - BAES had learned some hard lessons on Typhoon by this time).
In my view, concurrency was something that happened to the programme, but was not planned. With hindsight, they should have slowed the move to production more than they did. But then LM Fort Worth are an outfit who are dominated by their production people. (again, my own view).
The key point (at least in my view) is that the US has 'bet the farm' on the F-35. It will be made to work, and (again my own view) it will be a very successful aircraft, mainly due to its highly advanced sensors, communications, displays and weapons systems - things that (for obvious reasons) don't often get very detailed attention on this forum.
I hope these musings help a little - and I very much endorse the pleas from other (better) contributors for civility and patience. None of us know it all. Some of us know a little. Some know a lot (not me, by the way). On an open forum, we all get a chance to contribute. We can all learn. Bad manners and sarcastic points scoring will deter those (like me) who just try to help. Let's keep it nice, shall we?
Best Regards as ever those who are working the hard stuff in the free world under the scrutiny of a free press,
Engines