log in to my account on the MacBook while my wife is using it with her account, and to do this from my iPad.
That you can do. You need a remote desktop (VNC for OSX, RDP for Windows) type of app. Unfortunately, most of those cost money, often $15 and up.
I have and use Desktop Connect, although I find it only mediocre. I haven't personally tried any others since I really don't require that functionality too often, not enough to warrant buying potential duds.
Based on reviews I've read online covering many remote desktop apps, the next one I intend to try is Jump Desktop, but I'm waiting for it to go on sale (currently $15, but their price history shows they've dropped it in the past).
I've seen a lot of people that say they like LogMeIn, and it's free, but it requires setting up an account with them to use it. However, it doesn't seem right to me that I'd need to create an account on some service just for my iPad and Mac to talk locally on wifi.
I've also been intrigued by Screens, but reviews hint that it might not be quite as bug-free as it should be.