I made something similar to the "champagne" style you mentioned. I don't think you need to use a clear cider, it will clarify if you give it enough time. You may also add pectic enzyme if you start with a cloudy product. I like a white wine yeast such as EC1118 or redstar cote du blanc. I've also used nottingham and the white labs product (the latter I didn't like). I just saw a post where a wheat yeast was used (www.themadfermentationist.com
). You probably know this, but don't forget to add some yeast nutrient and ferment on the cooler side of ale temps if you can.
Store bought cider will be less tannic/acidic than something designed for hard-cider, but if you go high on the CO2 you will cover that up a bit. If you are going for "champagne" level CO2 then aim north of 4 volumes. For a wedding you want that nice POP when you open the cork, so going even to 5-6 volumes could be in order. For this you will need champagne bottles and a corker that can do the wider corks. I know some people that have been able to push the heavy corks into the smaller corker, which runs about $70 (get the corks first, bring one to the shop to try it out). I use the ferrari corker, which is more expensive but works great. Many HBS will rent them for a few days at a time; this would be the most economical way to do that. You don't need to use "champagne" corks, I would get the "belgian" corks from more-beer or elsewheres. They hold pressure in champagne bottles and are cheaper and easier to find. The champagne corks I got from midwest supply (or something like that) were junk.
If you are interested in the "methode champonoise" or whatever, there is a really nice review on the maltose falcons website. Watch out though, you would need to first cap the bottles with the 29 mm caps (make sure you bottles will accept caps, belgian bottles will not) then cork later after discharging the yeast. I've never done this, it looks hard.
For the keg version you may wish to try and backsweeten to increase the mouthfeel as you will have less CO2.
Higher OG = more aging/conditioning time and will thin the mouthfeel down some. As you know, cider will usually go all the way down to 1.000 or less, which ends up being around 6-9+% depending on your juice. Most of the adjunct driven ciders I've done did not taste as good as with the juice alone, but this is a matter of taste. Either way, I would measure the OG of the juice before you start and decide how much adjunct you want to add from there.