Do I need to constantly be wearing while the cavalry is being made for its time to be shortened? Or can I wear it just at the time I queue the production of cavalry to reduce the time and once the production is underway just swap to the regeneration helmet? By the way, does helmet of horseman work if the hero is dead?
You need to wear it when you queue after that you can swap. And it works only if hero is alive.
It also only works in the village that the hero is in so if you're running horses out of multiple villages the hero needs to move.
You could build a long que in the barracks using an infantry helmet, switch to the stables using a Calvary helmet and then leave a CP helmet on to help with growth.
Rotate the three.