Questions regarding chocolate imperial stout

Trust me. You are not asking for much.

I never used nibs, but presumably, if they are sweetened, they will add a little extra gravity that will ferment out and out more alcohol. It is probably not a big deal? I'd give it a try. As a guess, I'd nib it towards the end of primary.
I have used cocoa powder in the secondary but have seen it added at the end of the boil as well.

 

The thinning effect of sugar is most noticeable when you are replacing some malt in a recipe with sugar. In this case, you replace something that is incompletely fermentable and yields some body with something that is completely fermentable and yields little body. Unless you are replacing some malt to account for sugar, I wouldn't worry about it. Plus, your beer is a Russian Imperial Stout. If brewed to style, it will have a high gravity derived from plenty of malt, so it likely will have plenty of residual dextrins that provide body.

As I said earlier, I never nibbed, so I can't say exactly how long. In Northern Brewer's Chocolate Milk Stout (http://www.northernbrewer.com/documentation/allgrain/AG-ChocolateMilkStout.pdf ) , they recommend 4 oz of nibs for 2 weeks in secondary. Might be a good place to start. Wouldn't be a bad idea to taste your beer on weekly intervals and stop nibbing period according to taste.