1. You need more malt extract, 4 lbs is not going to be enough. You want at least 6 pounds or so of LME, I would guess. I would need to plug the numbers into Beersmith to give you exact numbers, which I can do if you want... but in any case, 4lbs will not be enough. ALSO, you will need to add more hops when you add more sugar. Personally, I would use an un-hyopped extract and do all the hop additions yourself.
2. You could use a bit more Chocolate malt, and probably add a little Roasted Barley as well. Chocolate Malt does not = chocolate flavor. Maybe a little bit, but not a lot. The name is a bit misleading.
3. Oatmeal cannot be just steeped - or, it can, but it wont do anything. It needs a cereal mash, which is a lot of work. I would recommend getting flaked oats instead, they have already been gelatin-ized. However, even flaked oats need to be mashed with some two-row, because they cannot convert themselves. Use 1 lb of flaked oats and 1 lb of (crushed) American 2-row. "Steep" (mash) at 154 for 45 minutes, then go ahead and bring to boil and remove around 170. (A better option would be to actually steep then in a second kettle of 170 water for an additional 15 minutes, the equivalent of a sparge - you will rinse more sugars from them).
4. From experience - boil a little water and dissolve the cocoa powder in that water before you add it in the secondary, otherwise it just hangs out and doesnt mix in as well.
6. Don't use Munton's yeast - that is a british yeast, it is going to give you some off flavors. If you're using a dry yeast use Safale, the American Ale (or California ALe) strain, I think Safale 04.
There is no reason to substitute brown sugar for corn sugar, it will not affect the taste. You could add brown sugar during the boil if you wanted, and add a bit of molasses flavor, but there is no reason to do it for priming. 4 oz - 5 oz should be OK.
You might want to add some Crystal Malts. I recently did a RIS and regret not adding just a little Crystal 40 and a little Crystal 120, to add complexity. And, if you add the 120 that might give you some of the flavors you're thinking about when you think of brown sugar.
Hope that helps.