What emulsifier will help my (nearly carb free) Milk Chocolate?
Yes, an emulsifier is the way to go. Lecithine is an emulsifier, and will work. The downside is that it might impart a slight eggy taste, I don't know if this will be a problem for you. Also, it is a bit harder to store than the other emulsifiers, it tends to lump from ambient humidity. 

The more common emulsifiers for your case would be xanthan or guar gum. They are not carried that often by stores, although I've seen guar in a health store. But they are quite easy to purchase online. Any of them will work well. The biggest challenge is to disperse them evenly in the food. I would recommend taking a teaspoon out of the cocoa powder, mixing the emulsifier with it (dry), then mixing that teaspoon back into the complete amount of cocoa, then proceed as usual. 

You don't have to fear the carbohydrates from emulsifiers. All three will work when used in tiny amounts, about 0.5% - so you can get away with 0.5 g emulsifier per 100 gr "chocolate bar", which shouldn't be much even if they are pure carbohydrates. (Xanthan and guar are carbs, but they are polysaccharides, and I have no idea whether they are insulin-active). 

This will help you make your chocolate bars. On the other hand, I don't know if making them is helping you at all. Cocoa powder is 60% carbs, and splenda is 90% carbs. A true chocolate bar (the good brands) consists of cocoa solids (which is cocoa powder + cocoa butter before being separated), and sugar. If you are using the same proportions as in a typical chocolate bar, you are getting about the same amount of carbs from your mixture. It may be a tastier way to temper pure chocolate (unsweetened chocolate) together with some fat until you have reached a carb-to-fat ratio sufficiently good for you. Adding cream is also a possibility, as well as other flavors (vanilla, orange essence, whatever you want - basically unsweetened truffle making). You can also add pure artificial sweeteners to your mixture, which are not bulked up with carbs the way splenda is. This way will be more expensive and time consuming, but will give you a better quality end product.