i have lots of suggestions (this may ramble and be long, so please bear with me - I'll keep it as short a ramble as possible! ;0)
first, I would switch to cloth diapers - second, your girl may be "sensitive" to the diaper rash cream - and thirdly, count the diapers. I know this may sound crazy, but if she says she is changing, then count how many diapers are missing. The reason I suggested cloth diapering is that she will have to change her more often because cloth isn't as absorbent as the chemicals in disposables. Then, you'll have a good idea of how many diapers she goes through in a day - you'll have them to take them home to wash - with disposable, I doubt anyone would "go through the trash" to see how many diapers were use. You could ask her to start keeping a "journal" like daycares are supposed to. They keep track of when diapers are changed and what happened, i.e. pee/poo - they also keep track of what they eat for snacks and lunch and how long or if they slept. Just a thought. Good luck momma! :)