I guess I'm looking at this from another perspective. Do you pay the nanny for any of the time she has off? Does she have a second source of income? The reason I ask is because you've mentioned that you'd like to give her a good chunk of the summer off. Does she need income during that time? If so, you may lose her to another family.
When we were looking to go part-time with our nanny, whom we also love, she said she couldn't do part time because she needs a full time job to pay her bills. We ended up finding another family wanting to do a nanny-share the other time she isn't with us. That has worked well for everyone.
As far as getting prepared, it sounds like you are right on track. Having someone help with your DD for a few hours of the day (or even all day so you can rest) while you take care of the baby is a good idea. OR have the nanny take care of the baby some hours of the day while you are out spending some one-on-one time with DD is a good idea too, so sounds like you've thought that part through well.
" I object to violence because when it appears to do good, the good is only temporary; the evil it does is permanent." Mahatma Gandhi
"This is the ultimate weakness of violence: It multiplies evil and violence in the universe. It doesn't solve any problems." Martin Luther King, Jr.