First off, welcome to the forum! Glad you found us.
Secondly, that's not a figure 8. It's a ceylon puffer.
Check it out:
http://www.thepufferforum.com/forum/ug. ... uviatilis/
Much different puffer than the F8. Can grow to 8 inches, require high end brackish to marine conditions as it grows, and requires about 40g to be comfortable. The 55g tank you have him in now is perfect for him. For his best health you'll have to start looking into getting some marine salt and a refractometer or hydrometer to measure the salinity. We can talk about that later.
As for your questions:
1. That's normal. It especially seems to be the case with ceylons, but all puffers have some colour changing abilities to show off their different moods, be it sleeping, hunting, attacking, sickness or stress. All will cause some colour change, and is usually normal if the tank is in good order.
2. The teeth don't look too bad from the pictures. Unless they are poking out or he has trouble eating, you still ahve time to keep them worn down with foods like snails.
3. Jumbo shrimp may help a little bit, but you really need to start feeding him snails pretty much every day.
Check this out for feeding ideas:
http://www.thepufferforum.com/articles/puffer/food.html
Setting up a little 2-5 gallon tank as a snail breeder is a great idea and very easy to do.
See here:
http://www.thepufferforum.com/articles/ ... snail.html
4. It is normal for puffers to lay around at the bottom of the tank when they sleep. If you provide a cave or over hang of some sort they will usually claim it as their home.
5. Are you sure there is no poop in there? Does his belly get sunken soon after eating?
6. How often and how much do you feed him? For his size probably ever other day or every 2 days would be a good schedule. How big is he?
You've got a great looking puffer there! I'm jealous! I always keep an eye out for ceylons that are actually mislabled as ceylons. They can be kind of rare.