Something you all might want to try for rinsing, which I learned when I was hennaing a lot: get a bucket and fill it up with water and immerse your hair it, swishing it around. This will get the henna out more easily, and with less agitation to the cuticle.

You might need to empty and refill the bucket a few times. It's also easier this way to see when it's all out, because it's very easy to see if the water is clear or not.

Happy hennaing/cassiaing!