Problem One:
I would nix the stilts just for the sake of ease and expense. You can try making your own, but I don't know con policy, and that sounds scary and dangerous. If nothing else, I've seen home made stilts before at Sakuracon, and it was clunky. Not what you would want with something like this. My suggestion would be to get a good pair of comfy heels from a brand like Sofft, Aerosoles, or maybe even Nine West or Franco Sarto. You might have to drop a pretty penny, but they're shoes you'll wear again more than likely. Anyways, so you've got your heels. Great. Now my real suggestion is to make boot covers for them out of the same material you would make the spike out of. That way, while visible, they'd be a little more natural looking that just bare legs in tennis shoes.
Problem Two:
A skirt might be a great idea! If you can find a stretchy fabric of the right color and texture, you can even make a figure hugging skirt that ends in a "V". Not exact, but it'd be easy.
If you want structure, you can layer up bed foam and shape it out with a bread knife and sharp pair of dressmaker's shears (scissors. They need to be sharp.) Form the foam so it will pad the back of your thighs, swoop forward, and point at the front of your shins. Cover with desired fabric, and there you go. This would be a lot more difficult than it sounds. Plus, bed foam is really messy, but it's really a pretty great material. It's flexible (in larger chunks to a degree), it's hot glueable, but it will be HOT.
Another simple solution is to make leggings of the color and texture of the spike and wear some sexy heels with it. That might be the way to go, actually. In fact, as much as I like accuracy, that's what I might do if I were to try this. McCall's has a bunch of legging patterns, and they go on sale at JoAnn's for $2 a pattern. I just saw the pattern this past weekend. Tada:
http://mccallpattern.mccall.com/m6173-products-11288.php?page_id=114 I wasn't kidding
Problem Three:
Sounds like a great idea! But, make sure you have someone down pat to help you out with that. That cosplayer's hair probably took at least an hour and a half to do. If someone agrees to help you, they better not let you down because you'll be stuck in a possible but highly tricky spot without them. I would venture just for the styled wig, mysefl. I would also invest in a good box to keep it in on its own so it won't get squished. I would be SO sad if that happened to you. Seriously. Tragedy.
Problem Four:
I think these hands are highly and fully possible. I suggest making them as hollow as possible to lighten the load. You can use bedfoam for basic shaping and Rigid-Wrap (plaster coated fabric strips available at Michaels or JoAnn's) over it for durability and structure, but you won't really be able to articulate anything. For articulating the hand, you'll probably want a structure like this guy:
Possible, but tricky. If you know someone who's good at rigging stuff like this up, go for it! It'd be so cool! Make the fingers woodend and just put some fabric over it. Sure it might not look perfect what with the rigging underneath, but no one will be paying attention to that if they move. If you're really worried about it, add a little bed along side the rigging, and then cover with fabric.
For fingernails, easy enough, I would think. Craft foam with red metallic posterboard over it or you can use RIgid-Wrap or Sculpey with a foil and wire core for durability.
Problem Five:
Back spikes? Pish posh. Make them out of craft foam or cardboard and have nude sheer fabric across your back. Attach the spikes to that and voila. You just might need clear plastic bra straps (Joann's again) to really keep your top up. If you want to try something a little more daring, make them out of something really light and glue them onto your back using Spirit Gum. Just be sure to buy Spirit Gum remover or your back will be sticky for a while.
Problem Six:
Fake thighs: SHape them out of bedfoam (seriously this stuff might be hot and totally messy, but the it's so versatile!). For poof you can buy a red or black petticoat (There's a place near the Multnomah County Library that sells petticoats. It's a vintage shop. I'll have to get the name of it, but I remember seeing a lot of poofy petticoats there) or you can try using chicken wire. Chicken wire is great for making shapes, but it runs the risk of bending (although relatively easy to reshape) and it might poke you a bit so wear junk leggings over your tights or whatever so the wire doesn't cause a run.
The stomach, tough, well, you can try a nude sheer piece of fabric in the hole of the dress, you can just cut out that part and edge it, or you can create a fake belly. That's up to you.
For simple dying solutions that might, just might, work for your outfit, you can test spraypaint to see if it looks right. Whatever you use, dye or bleach, I would put it in a spray bottle and spray the dress over a drop cloth. You can try dipping the ends of the dress of course, but that will cause a greater mess. Probably a cleaner dye, though. Test out whatever method you try first before moving on to the real thing.
These obviously aren't fool proof methods of making that crazy, crazy outfit, but hopefully it'll give you some ideas on how to go about it. Post your progress on the cosplay progress forums if you get the chance to show this off!