Onions give off a lot of water, so this process would end up steaming/boiling them more than sweating them. Caramelized onions, in order to reach their peak taste/texture, need to sweat. I find the best flavor is achieved by sweating a single layer, but... if you work with a higher temp, use plenty of fat and keep the onions moving (preventing steam from building up too much), a few layers can be forged into a stellar product. If, in the past, I needed a large amount of caramelized onions and didn't have the time to do multiple batches, I've gone as thick as 2", but it wasn't without a considerable loss in quality. In a slow cooker, I can only see this loss being even more dramatic.
I agree with your recommendation to avoid caramelizing sweet onions. The end result is sweet, but pretty much tasteless. Shel, ideally you want to find the most pungent onions possible, as caramelizing them tends to mute their flavors a bit. Onions seem to have lost a lot of their punch in recent years. I use to cry all the time cutting onions... these days they're so wimpy I'm lucky if I cry once or twice a year.
I sweat my onions for a very long time- sometimes as long as 2 hours, depending on the moisture content in the onion/toughness of the cell walls. With a very long sweat, I lose a ton of water. I can't say for sure about the beginning/end weight, but the final volume is, at most, 1/6 of what I start with. In the context of your 'liter' question, I'd say purchase the equivalent of 6 liters of chopped raw onions. And, like Castironchef recommended, make sure you have an extremely large pan (or do them in multiple batches). Once you get the technique down, you can experiment with a few layers of onions, but for the first time, I'd sweat a single layer- and don't be stingy with the butter.
As far as which fats to use, it really depends on the recipe. If I'm caramelizing onions for meatloaf or for steak au poivre, I use beef fat. If I'm making tomato sauce, it's olive oil. For sausage w/ pepper and onions- pork fat. Both pork and beef fat tend to be pretty awe inspiring choices for caramelizing onions. Other than making roux for gravy, I don't cook much with rendered chicken fat (schmaltz) but I'm sure that's equally as earth shattering.
I'm not a big fan of adding ingredients like sugar or balsamic vinegar to caramelized onions. If done right, they have plenty of sweetness on their own. If I was going to add balsamic to my onions, I do so in the final dish.
I've frozen caramelized onions two ways- in ice cubes and in a flat thin layer. Ice cubes worked alright, although it's a little too much surface area for my taste. Remember, we're dealing with a very high fat end product, and fats (especially butter) tend to absorb freezer odors. The flat thin layer (wrapped in plastic) was a little better in this regard. The nice thing about the layer of onions is that a piece could be broken off quite easily.
In the fridge, I've stored them successfully for about a week, but I wouldn't recommend going past that.