Since the limitation of cable drive, I am considering a gear which is backdrivable.
Is crossed helical gear backdrivable?
Also where can I purchase miniature crossed heilcal gear online?
you can backdrive helical gears provided the helix angle is not small. Say less than 8 degrees for static driving and less than 3 degrees for dynamic driving. Now, when you say crossed helical gears, you got to be careful what the shaft angle is. In one case they need to be same hand and other they need to be different.