Personally I never found Lactose (also called milk sugar) to do much sweetness wise, but then again I found out years ago I was lactose intollerant. No wonder all those Milk Stouts treated me so badly...

There's some tricks you can do to help the fermentation of a high % brew. Someone mentioned using two packs of yeast, but you can also try
a) hydrate the yeast first in clean distilled water first before adding. Sprinkle on the top, don't stir. The dried yeast wall is unable to filter the initial fluids when it's re-hydrating, leading to the yeast taking in all kinds of stuff that doesn't normally exisit inside the yeast cell. This can make it die prematurely

b) make a yeast starter - boil and cool 1lt of water with a couple of tablespoons of dried malt (get it at the bewshop). Aerate well, and add the yeast. Let it ferment for a day or two - you are in effect slowly introducing the yeast to a weak wort mix which lets them breed up lots more healthy yeast cells. Pour it into your high sugar content beer and they'll have a much better chance of fermenting it out before conking out. Don't worry, you won't be diluting your beer anything noticeable.