Do you use bubble bath? My DS HATED bubble bath, he only liked bathing in plain water!

Or alternatively, make it a bubbly bath or use one of those bath gels that changes the colour of the water on hairwash night.

As for the hair...maybe you could comb out all the knots using a plain white conditioner (like you would for nits). She doesn't have to be in the bath then, and then you'd only have to have her in the bath long enough to rinse it off. If you load up her hair with conditioner, it could be a work in progress over a couple of hours if you can't get her to sit still long enough to sort out the knots in one sitting. Put her in front of the TV maybe, while you work on it.

Would she like playing with one of those hand held shower head hoses that you attach to the bath hose?

ETA: DS went through phases where he suddenly didn't like the bath and wanted a shower, and then one night all of a sudden he'd want the bath again...that would go on for a while and then we'd be back wanting a shower again. No warning.