Well, the concept of Malech came to exist after Indians came in contact with the Caucasians who came visiting on Ships from Europe. A caste-ridden society eager to prevent miscegenation (mixing of races)was quick to label them 'Malech' (dirty one). There is also a parallel belief that it was a practice related to personal hygiene - wiping instead of washing - that earned them that name. Maybe it was a combination of both these reasons, but they ended up being labeled as 'Malech'. That is a fact. The time frame of this was the 1700's to 1800's.
Its not uncommon to see sikhs hindus and folks from other faiths taking part in muharram procession, however i think its less to do with show of solidarity and more out of their own passion or convictions. I know there're sikhs here who do matam religiously every year during muharram and hindus that lead taziyahs and hold thier own mannats for Imam Hussain and Hazrat Abbas...etc.
Look up hussaini brahmins and the festival of hosay, and you'll get an idea of how various folks commemorate it in their own way.
well..in bhopal..during the sikh gurpurab, the procession used to pass through the old city..( the city has two parts basically, a fort area and the newer development) and it was mostly muslim population there, but all the shops used to be closed that day as a sign of respect and every row of shopkeepers had a welcome committe setup and all..so it was pretty cool. People know how to respect each others religion. same thing used to happen during roze, the shops were allowed to open till late at night ..otherwise they was a certain time limit.
it started of as a political thing by hindus to curry favor from shia muslim nawabs dedicated some taziyahs during muharram processions.
Many shia clergy actually protested strongly against participation of pagan hindus in muharram but they were sidelined by nawabs who needed hindu support in a state where muslims were a minority. some nawabs also tried to end it as well
later some hindus might have formed some devotional cults and who knows maybe elevated Hussain b Ali [as] to the status of a god.