Russ - I recently made the dark ale and it has been in the bottles for 3 and 1/2 weeks. I'm finding that if I keep a couple bottles in the fridge to get them nice and cold they seem to be a little flat. But if I open a warm beer or one that is slightly chilled it has a nice head when pored and has great flavor.
BMaster - Not surprising that it shows more carbonation when warm. CO2 comes out of suspension faster in warm beer than in cold. After 3.5 weeks you should have good carbonation, but sometimes beer needs a little help.
My suggestion would be to shake up your bottles, then store them on their side for a week at room temp. This helps to get the yeast in the bottles going again and finish the job they started. You will likely find that the carbonation level when chilled is much improved.
BMaster on Increasing Alcohol Percentage-
The alcohol percentage of a beer you make is governed solely by the amount of fermentable sugar you make available to the yeast to convert into alcohol. The more sugar, the more alcohol. The catch though, is sugar in what form? You don't want to add too much brewing sugar, or your beer will be higher in alcohol, but will be compromised in body and flavor. My recommendation would be to add an additional package (1kg) of Coopers Brew Enhancer 2. This is a blend of dry malt extract (dme), brewing sugar, and maltodextrine. The dry malt extract (sugar based on barley) and the brewing sugar are both quite fermentable, with the DME also contributing flavor components and the maltodextrine contributing body. Adding one box of Coopers Brew Enhancer 2 will add just over 1% ABV.