I think the straw like feeling is because henna opens up the cuticle to deposit the color inside the hair shaft. (At least I think that's how it works). I've foudn that rinsing with loads of conditioner, leaving conditioner on the hair, and loading it with coconut oil (so that it looks "greasy" for a day or overnight) gets rid of the straw feeling really fast.
The red will darken but will still be red. Hopefully it will be cool looking red "highlights"