I understand your frustration but that was a minor spelling mistake as everyone has occasionally missed a letter on a keyboard and when spell check doesn't pick it up we can get lazy about rereading.
From the reply you were given it is clear that they either didn't read or didn't understand your question and I do think they should have done a better job, However your email was quite difficult to pull apart to find the questions. I would suggest a few paragraphs separating individual questions to make it easier to understand and therefore answer.
In the original email its a little tricky to tell what your asking and its very possible the person reading only saw the question on assigning characters from the expansion pack and assumed you would just do as instructed from documentation and let everything run its course as normal which is poor service but understandable
After reading in your follow up paragraph that your concern was not with the assigning of characters but with people knowing which characters were "extra" characters it was easier to go back and find new meaning in your original email, the confusion of multiple sheets could easily be sorted out on the night but sharp individuals would immediately notice the appearance of "extras"
I think the designer of the game thought of the extra characters in the same way as you would with innocent bystander characters where its immediately obvious they are playing a detective roll not a potential murderer and possibly they wanted to keep the potential murders down to the 20 to make sure the game was not too challenging, I know that even with only 20 suspects my guests found it tricky to pick the murderer.
It also becomes tricky because there are 3 possible expansion packs for Murder at the four Deuces, to include a character list of 30 would imply that you can only add 1 expansion pack to the game or if you added more per expansion then it would mean you couldn't add characters from expansion pack 3 without first adding packs 1 and 2.
I think this matter should be made clearer in the original documentation but i think its fine the way it stands where each person just gets the list of the 20 potential murderers and the extra characters get introduced on the day. The option of keeping people in the dark is good however i do respect it would be a fair amount of work to collect and include various pfd's for all the different scenarios possible.
As the Host you could always only send out each person individual character with no list of anyone else at all, keeping everyone in the dark and giving everyone a fresh reading of all the characters in play just before the event. This would solve this problem and only need the inclusion of character pdf's the only include their bio. (something to host could also make using the current materials)
Thanks for reading this far and hopefully this was some food for thought
