Re: minimum weight
"I was reading an article a while back about that way of thinking; apparently, there is a real problem with civilians flying ex-military aircraft in that they really don't want to eject."
In many countries ex-military craft have the ejector seats removed so a civilian pilot can't eject even if (s)he wanted to (explosives being illegal in a non-military craft, etc). There's also the issue that being in the aircraft means that pilots tend to take more care about where it ends up when things go ultimately pear-shaped.
"I think that might have been true for very early seats, but that was a long time ago. There's been lots of development on ejection seats."
Indeed there have, but a 1 in 4 chance of permanent disability is still far too high for my liking. There are quite a few pilots who've ejected once and are still in cockpits but very few have ejected twice and been able to resume active duties.
WRT ejector seats and head clamps: Older ones don't. Newer ones do - along with the also-mentioned airbags to securely clamp the pilot in for the duration of the (short) ride. The issue at point in the F35 is excess helmet mass and I'm pretty sure that the pilot mass requirements are a kludge answer as neck strength and crush/whiplash resistance are wildly variable even for people at the same mass and fitness (Think wrestlers vs footballers)
Presumably the problem could be solved in the short term by moving as much of the mass of the helmet as possible somewhere else but dangling umbilical cables are going to be problematic and it means the ease of dropping a pilot into any available aircraft will be compromised (plus it's something else to go wrong in the field).
Longer term, lighter components will probably be available but by that point piloted craft will probably be obsolete.
In any case the F35 was never designed as an air-combat/air-superiority machine. That's a job for the F22. The F35 is too tubby, too underpowered and with too stubby wings to be an effective fighting machine(*)(**) and by the time it gets into the field its much-vaunted stealth(***) capabilities will be so hopelessly compromised by countermeasures that most of its armaments are likely to be on external hardpoints as there'll be no point hiding itself.
(*) Because the F22 will never be sold to other countries you can expect the Eurofighter and other aircraft to keep working alongside the F35 for a long time to come (assuming it isn't cancelled - and even the SDI got cancelled in the end on cost grounds despite massive multistate pork-barelling)
(**) These same features make it badly compromised as a ground-support aircraft. Short range and low carrying capacity mean it's less capable than what it replaces.
(***) F35 is ONLY stealthy on a 30 degree cone around the nose end of the longitudinal axis(+), ONLY stealthy at currently used SHF radar frequencies(++) and ONLY stealthy with the doors closed(+++)
(+) Networked detection systems can see it clearly from side or rear and transmit to AA batteries located along the flightline. Newer AA missiles can be guided from those side/rear radar systems rather than relying soley on the onboard smarts or controllers near the launch site. F22's role is to eliminate this threat but other countries won't have the F22
(++) Painting with VHF/UHF radar has been shown to work (UK air defence can see B2s even when they're fully stealthed up) and past US stealth tech has failed hoplessly when the wings are wet (which is how the F117 got shot down). Water is a pretty good reflector so all the matte paint in the world may not help much.
(+++) Neither of the above points matter much when you have a "stealth" aircraft so badly designed that the weapons bay (and other) doors need to be opened in flight every 10 minutes or less due to the avionics overheating and needing emergency cooling. At that point it'll cause even the dumbest radar systems to light up like a christmas tree.
The F35 is one of the most expensive clusterfucks the USA has ever engaged on and it's going to fuck their economy even worse than Ronnie RayGun's Star Wars program (which left the USA deeper in debt than it had ever been at any point in history). The best thing that any other country can do is hedge on purchases (many have already cancelled) and try to make as much money out of the program as possible in the meantime.