When sizing up a skin for a ski you want as much coverage as possible. With that being said, we're working in millimeters, a very small measurement, and most of the traction you get when skinning up comes from the last 3/4 of the ski. The tip really doesn't help generate any traction. A 115 skin will obviously get you great coverage underfoot, and in the tail, you'll have 2 mm on either side of the skin, which is basically the thickness of the edge, and material you would have trimmed away anyway if you were cutting the skin to size. And in the front, you will end up about 5 mm on either side, hardly a large amount.

It appears that by getting the 115, you end up with an almost perfectly fitted skin without having to do any trimming except in the middle part of the ski to fit the waist. If you went for the next size up, you would be spending more money and cutting away a lot of material. The 115 is the perfect size. Go for it.

Here is g3's guide for choosing a skin. You will see the 115 fits in perfectly for the size ski you will be using: http://www.genuineguidegear.com/service/g3-learning-centre/skins

Tricky the mediums range from 171-181 and the Large 180-190. I think you would be able to get away with the Medium you will have some tail showing on your larger skis but that should matter since they are twin tipped and the tail doesn't even hit the snow.

Yes you will be good to go!

Yes, that is what they specifically designed the new tip/tail attachments for.

Def yes on the trim tool.
I got last year's and they came with Savers too, so I'm pretty sure that's a Yes too.

Trim it to the point to where just the edges are showing.

Get the skins that fit the widest part of your ski (except for some extreme fatties) and then trim down to the waist. You'll appreciate the extra coverage.

Dustin, these skins have a set length bracket since there's no folding through a tip loop to adjust for your skis; that's why they come in S, M, L etc.

Pick up the xShort length. Get a width that is close to the widest part of your ski, then open a beer. Trim the width with the included Skin Cutter. It's super easy.

x short 153 - 163 70, 85, 100, 115, 130, 140 411g / 14.5oz - 656g / 23.2oz
Short 162 - 172 70, 85, 100, 115, 130, 140 431g / 15.2oz - 694g / 24.5oz
Medium 171 - 181 70, 85, 100, 115, 130, 140 451g / 15.9oz - 732g / 25.8oz
Long 180 - 190 70, 85, 100, 115, 130, 140 471g / 16.6oz - 769g / 27.1oz
x Long 189 - 199 70, 85, 100, 115, 130, 140 492g / 17.3oz - 807g / 28.5oz

you buy skins based on the width of your skis not really the length as the skins are cut to fit the length of the ski by you. You can either measure the tip of your ski and buy skins that wide or measure the middle of the ski and buy that width. If you buy skins based on the middle width you won't have full coverage and may slip in icy conditions. I always buy skins based on my tip width.