slippery elm vs. marshmallow

I was looking a tea for the throat, and i saw this mix from Hv

Mix two parts of slippery elm with one part of licorice root.  Boil one teaspoon of the mixture per cup and a half of water with the pot covered for 20 minutes.  Then turn off the heat and let it steep until it cools.  Use the tea to gargle with and sip on throughout the day.  The slippery elm will coat and soothe the throat and the licorice root will reduce the inflammation.

Then i went to an herbalist near my home but don't have slippery elm and he told me that marshmallow have simillar propieties

my question is, will work the same? and would be the same amount mixed? marshmallow 2 : 1 licorice?

is the marshmallow leaves or root to use?

thanks a lot