Alrighty, this is a pretty complicated answer so I'll try my best to provide a straightfoward narrative to it.

First, there are two words in question here: hashish and hashishi, the former being obvious in definition and the latter being something more like how one would use the word "junkie" or "scum" in modern usage as a derogatory term. As you can see there's very little that separates the two words on a phonetic level (though not much definition-wise) and this probably contributed in a great deal to the back and forth throughout historical scholarship on the subject.

Second, the prime reference anyone has for hashish being the origin of the word assassin is from the work of reknowned French philologist Antoine Isaac Silvestre de Sacy. Now, his knowledge of Arabic was very substantial being the Professor of Arabic that he was but there's really little evidence to suggest that hashish connection as the first knowledge we even have of the term for hashish comes nearly two hundred years after the Assassins.

What we do know, according to the wonderful text of Modern History of the Ismailis, A: Continuity and Change in a Muslim Community by Farhad Daftary, is that the term hashishi was used as an insult and a derogatory term for the Assassins. Daftary asserts that those unsympathetic to the Islamic cause resorted to this term often, contributing to the modern theory that it, or its phonetic sibling hashish, contributed to the name of the Assassins.

Regarding the explanation you've sourced from Wikipedia, I can only note that it was written by Amin Maalouf. I personally have not read the work that the quote was taken from but I can vouch for his scholarship as his non-fiction book, "The Crusades Through Arab Eyes" was informative and illuminating so I would take his opinion with more weight than I would the reddit poster off the street such as myself.

EDIT: This answer should be compared, contrasted, taken into context with, etc. the knowledgeable additions of /u/jankyalias and /u/GrandpaDave for a more comprehensive understanding of the subject.

Regarding your second point, my reading of your post is that you are suggesting a fervently religious person would not be likely to encourage or even allow his followers to consume drugs. You can discard this line of thought completely, as all the way up to around 100 years ago it was completely normal for people of many religions, including Islam, to encourage the use of hash (and many other drugs) to aid in worship or spiritual journeys of enlightenment. And Hassan-i Sabbah was certainly from the area and time period that this was most commonplace.

I've not read it myself, but the source I've seen mentioned several times when the 'Booz Rooz' (the name of such ceremony) is mentioned is Abu Usaybia, Uyunu al-Anba fi Tabaquat al-Atibba, Berkeley: University of California Press, 1965.

So actually it's more likely that a religious person would be highly encouraging of the use of hash.

Be careful with the chronology of events when considering 'pro-Crusader' smear in the context of Hassan. He was already installed and active in 1090, the best part of a decade before the Franks arrived in the East and was dead by 1125. Many of the Assassin stories instead come from later, when the Franks knew their leader Rashid as the Old Man in the Mountain. It was this later flourishing of activity that some chroniclers like William of Tyre have focused on, in the late 1160s for example, when the Crusader-Assassin alliance was in its formative stages!

Was there an expression pre-Assassin for someone who performed the same duties?