Dry skin can itch, so can a scalp covered in residue, say from the conditioner. Do you rinse your scalp for a long time after you wash/condition? Rosemary essential oil can help with dandruff and itching. How about doing an ACV rinse? Get a basin and put about 1/4 cup of apple cider vinegar. Fill that basin/bucket with water (about 2 gallons or so) then add 20 drops of rosemary essential oil. Dunk you head so your scalp is submerged and massage it under the solution. I always do this at the end of my wash and my scalp tingles and feels so clean. Also take a supplement of good oils like flaxseed or fish oils. They'll help with any dryness you might have. Drink lots of water too.
There was a time I thought dandruff was caused by dry skin but oily skin gets it more. I know if any products get on my scalp, it itches like crazy. So when my hair's braided, I don't apply anything but I baggy every night to keep moisture from escaping. Maybe it also encourages the release of my own sebum so my hair never feels dry. And neither does my scalp. And not having artificial products on my scalp stops it from itching. I don't know if you'd be comfortable with that, but if you only have a short while to go, why not try it.
On the day you are to undo your hair, maybe you can apply a natural oil or conditioner to provide slip to undo your cornrows. But with regular bagging, I find my hair doesn't feel dry. It is ages since I wore cornrows for long, but I seem to recall some people finding sulfur pomade/grease helped with he itches. Coconut oil too might help. It's one of the best moisturizers so it will take care of dryness if that's your problem. It is also an antimicrobial remedy so if the itches are fungal, it'll help. In other words, it's a safe product to try with no side-effects.
