Hey OP, length retention out of braids starts with taking care of your hair while it is braids.
You said something about not being able to DC for 2-3 months. You can DC while in braids. If you go to www.growafrohairlong.com
you can get some great tips for taking care of your hair while in braids. The truth is treat your hair while in braids the same way you would if you weren't in braids. This makes length retention so much easier.
But to answer your question. You spoke about getting the dirt out with a comb, you dont have to be rough when you use the comb. You could saturate your hair in conditioner and then take it down slowly.
Do you have a shower comb?
Do you have seamless comb?
These two types of combs help reduce breakage as well.
There is also some spray that helps take down braids, this spray helps break down the build up. The build up is also a problem.
Do you wash your hair at all while you are in braids?
Hopefully you do. Again, if using the above the site will give tips about how to wash your hair. This helps reduce build up.
I hope this helps. But remember the next time you get braids, treat your hair the same in and out of braids.