Hello Brian
Welcome to the forum. I've been going to Le Mans for more than 20 years, but had to miss out this year because of work. I'm English, but live in the USA.
Opinion may vary slightly, but here's the general skinny on your questions.
1. If you're fling into Paris (as we do) rent a car - it's much easier than dicking around with trains - and you will have transport once you get to the circuit. Car rental is a little more pricey than in US and gas prices are scary, but if you get a deisel car, you won't burn horrifying amounts of fuel. The drive from Paris Charles De Gaulle to Le Mans is 3.5 hours - all highway.
2. Yes - there is a bus service between the circuit and Le Mans - but you'll have a car, right?
3. Sebring and Daytona are my home circuits. Sebring has some parallels with Le Mans both on and off the track. Both have a big party atmosphere and lots of history. Parking/camping at Le Mans isn't the free-for-all that Sebring is. You will need a parking/camping permit and you should apply for that (from ACO) as soon as you've fixed your plans. Many on this forum favor the Maison Blanche site (MB), but there are many (I prefer Houx personally - but it's all down to personal preference). Facilities at Le Mans are a little medieval compared to US tracks, but it's a small sacrifice.
Even at this point your choices may be limited, so don't leave it too long before you book your pitch. Yes - there are 'diamond screens' at a couple of locations - but they are trackside - not in the camp grounds.
4. Mainly on foot - so don't bring any fancy footwear. Sports shoes or hiking boots are what you need. there is a shuttle bus from the more distant camp grounds (Beausejour and Technoparc). There are two restaurants on the Hunnaudiers straight - but they are over-rated, overbooked and you can't see squat from either of them, so I would advise you not to bother. Driving your car around the local roads during the race is a pain in the arse, so I would be inclined to buy all your gear before you set up camp. This way, you won't get busted for DUI either, which will ruin your whole trip (and you'll get to see what French jails are like!)
5. Stay in touch and introduce yourself to the regulars - most are very generous, helpful and friendly. Get yourself to Road Atlanta for the Petit and meet a bunch of us who do this race as well as Le Mans and the others you mentioned.
Just holler if you need anything - there's always someone with the answer here.