1. The toasty bitterness is likely from the roast barley, and to a lesser extent the crystal 140. As you're wanting to increase the sweetness, maybe leave the crystal alone and drop the roast barley. There's not much there to begin with, so I wouldn't drop it much below, say, 70-80g as this would be only lightly discernable in a beer (other than darkening the colour). Keep in mind that dropping the roast barley will make the malty sweetness from the crystal stand out a little more.
2. If you were mashing, I'd suggest mashing at higher temps to achieve what you were after. However, seeing as you're doing extract, adding carapils would help. Also try a different brand of extract, as no two brands are the same. I can't help you with which to choose, but I'd assume an English brand of extract (if you have access to it) would have more body. Otherwise, yes, try CaraPils.
3. Maybe bump the CaraMunich I up a little for sweetness. But remember that all these things tie together. Firstly decreasing the roast barley will increase the impression of sweetness. Secondly, increasing the CaraMunich will increase the body of the beer, meaning you may not need to add as much CaraPils as you'd think. It's all a bit of a juggling act and some guesswork needs to be employed. Perhaps up the CaraMunich to 300g for a touch more sweetness and body.
As far as judging quantities without being over the top, there's no real way to do it except by brewing lots. Firstly, this allows you to tweak your recipe to the point where you're happy with it. Secondly it gives you some experience with the different ingredients, and gives you a knowledge of what they contribute to the beer. Brew it 3 times, upping the CaraRed, CaraMunich and Crystal 140 respectively and then compare the results.