PhilipG,
Perhaps I can help here.
The first point to understand is that putting a capable cat and trap aircraft (like F-35) on to a ship is not just a matter of laying down catapults - the arresting gear requirements are also very demanding. The ship also needs to be big, fast, and able to carry the fuel and weapons required to support the desired sortie rate. The USN's experience over the past 40 years (and it's being borne out by the UK and China, as well as France) is that a ship design under around 65,000 tons is going to struggle to deliver a meaningful (i.e USN meaningful) cat and trap capability. It's probably going to need to be nuclear powered (mainly to provide the power required at an acceptable volume without having to trunk air down and exhaust up through the ship).
Bottom line (in my view, but happy to explain further) is that LHDs would definitely not be a practicable cat and trap platform. Too small, too slow and not enough internal volume (those tank decks and dock wells take a lot of space). (By the way, EMALS requires more volume than a steam catapult - and a whole lot more electricity). The books by Norman Friedman are an excellent reference source on this complicated matter.
As I've posted before, the genesis of F-35 was a series of failed US combat aircraft programmes. The people in the Pentagon came to the conclusion that the next attempt had to be a single seat single engined aircraft, so as to contain cost. At this stage, people will be jumping in and pointing out how much the F-35 has cost, and they'll be right to do so. However, that was the idea.
The next step in the Pentagon's plan was to make sure the design stayed single seat, single engined (because the USN really wanted a twin engined aircraft) , and they did this by adding in the STOVL requirement. At the time (early 90s) and even now, there are no feasible twin engined STOVL combat designs out there. STOVL adds in a powerful discipline to keep weight down (which LM went and forgot) and lower weight keeps down size, which keeps down cost.
The USMC had been working on advanced STOVL for many many years, via various open and 'black' programmes. The Pentagon planners just took in their STOVL Strike Fighter (SSF) requirements and added them to what was becoming JAST/JSF.
My view is that if the STOVL requirement had not been added to F-35, it would have been a very different outcome - and probably more expensive than the result we see today. But I recognise that many on this thread would differ. That's fine - it's a free forum, and all the better for it.
As far as the USMC's commitment to STOVL - I'd say total. It's long been a core ambition of theirs to deliver the capability to their amphibs and also to forward bases. As Spaz pointed out, these plans are based on a lot of doctrinal development, as well as the basic fact that the Marines want their close support aircraft, well, close, and under their control. They have a collectively scarred memory of failure by other services (USAF, USN) to deliver the support when and where required, and they are not going to surrender their ability to give the marine on the ground what they need. I had the pleasure of working with them for a few years, and it's vital to realise that they are not remotely interested in 'Air Power'. Every ounce of USMC combat capability is aimed at supporting the Marine on the ground.
Again, there are plenty of people who disagree with this vision, doctrine and approach. Fine. But the USMC have, over many years, fought their corner with skill, persistence and some cunning. They know where they are going, they make their arguments consistently and clearly, and the politicians support them.
Best regards as ever
Engines