I made one attempt to run Masks before a couple of years, but it didn't go past New York; Now I am to start Masks, online, with 3 players, all seasoned roleplayers but with limited experience with CoC.
Reading around for tips running Masks the one that pops up most of the time is the "run prequel adventures solely to introduce Jackson Elias and have the party bond with him"
What worries me is that should I have a prequel adventure, just to introduce and have the PCs bond with Elias, this would mean that the PCs will start the NY chapter kind of seasoned with the occult and the Mythos. My feeling is that its best that the characters are green as far as Mythos is concerned when they start in New York.
On the other hand the prequel adventure could be non-Mythos related. It could have to do with some random non-Mythos cult-conspiracy. It could even be totally irrelevant with the occult (I gave some thought on the PCs and Elias being war buddies and the prequel being simply set in the butchery of Verdun). But in that case such an adventure will be totally out of the Call of Cthulhu sphere. Is it so much worth it to spent a couple of sessions in that manner?
In my last two (non-CoC) campaigns instead of having the players write up their backgrounds I ask them a general outline of what they want to play and then I create a questionnaire with 15-20 multiple choice questions about their past (eg "You decide to go to a voyage to a far away land, you decide to go to a) place 1 b) place 2" etc), with each decision they make granting them something and at the same time forming their past. My initial thought is to use something and have Elias being a part the most crucial decisions in the questionnaire.
TL;DR Is it worth it to have an entire adventure just for introducing Elias Jackson? Won't it be a problem if the PCs start the NY chapter as seasoned investigators vaguely aware of the Mythos?