I've never used the Ergo, but I can tell you about my Kozy...
1. We nurse in the Kozy all of the time...sometimes it's the only way to get my little 12 mo. old guy to concentrate and let himself relax, nurse, and fall asleep. I partially untie the front knot that's under his bottom and the re-tie looser to drop him down. I can then shift him up and tighten by myself easily even when he falls asleep nursing. (BTW, it helps if you anticipate the nursing BEFORE you tie on the waist strap and put on the carrier because the strap needs to be under your shirt if you plan to lift your shirt for nursing.) We've walked around shopping and nursing before and it's pretty discrete...it would be even more so if I wore a nursing shirt so my side didn't show at all
2. I always put Reid on by myself. It's easy, even the back carry. Kelley (maker of Kozy) has directions for putting them in the back carry two ways, but I always do it standing and have no problems. I can even shift him from hip to front without taking him off (or waking him) and have shifted him from my back to front both with and without help (probably not recommended).
3. My little ds is only about 17 pounds, but he's very comfortable to wear front or back. I don't like where the straps hit my neck in the hip carry, but we do it for short periods of time (it feels like the maya wrap hip carry around my neck only his weight is more on the waist).
4. I think a lot of his weight is on my hips, but it depends on how tight you tie the knot under their bottom. If I'm wearing him for a long time (sometimes for a 3 hour nap) I tie the knot across his back after a while so that all his weight is on my hips. And in the back carry you can switch from back-pack style straps to across the chest straps (really not flattering for your chest
) a couple of times to shift the weight. All that said, my back's tired after holding Reid for many hours at a time multiple times a day, but I'm sure it would be more tired if I didn't have the Kozy!
5. Kozy does all three. Additionally, I love how I can have him low in the Kozy so that he can nap and his head is even down in there OR I can have him high so that his arms are out. One note about the back carry, Reid never has enough head support once he falls asleep so his head just droops backward. I think it's because if I put him lower in the Kozy when we first put it on and he's awake he will balk because he can't look around.