Hi fire...
yes, you can clone the broken branch
The best way -
Make a diagnol cut about 1/4 inch below a set of nodes.. remove the branches coming from the node... dip into rooting gel/powder and then insery branch into rockwool cube..
if you can get somw Wilt Spray, you'll be golden!
ut the clone under a CFl... about 5-10 inches distance will be fine...
as far as the root rot..
1. make sure you clean out all deadplant matter in your reservoir... dead plant matter eats up tthe 02
2. add an air stone to your reservoir..
3. lower reservoir temperature
cheers