1. Not sure exactly when, but it was reasonably early. If your DD likes sitting up it would be fine to do it already.

2. The straps are too big to start with. I just made them as short as they would go and do them up. THey did look ridiculous but he never fell or climbed out. DH only used to do up the bottom one. I think at that age the one between the legs is the main one so they don't slide out.

3. It's very hard to do up from newborn to sitting when they are in it, but you can do it. You have to do the bottom buckle first from memory and it helps to have another set of hands to help. But I have been able to do it myself too (after some practice).

I hope you work it out. We now have the toddler attachment as well and I think it's fantastic.