The party doesn't need a wizard, but it doesn't hurt to have one. They can do the reading! Max had to take this role because Mortus wouldn't go into the sewer.
One thing you do need is someone with healing skills. I forgot to tell you this at the start. So Max eventually learned.
After all a level L4 necromancy [if my understanding is correct] is a 8 wizard [4 normal levels and four necromacy levels]
Not quite right. You have to be a L1 normal wizard first. You can then become a L1 specialist (illusionist, elementalist, necromancer, demonologist). But you don't have to take further normal wizard levels - you can carry on to the next specialist level.
There aren't any L4 wizards in this campaign! At all (no, Something Rotten in Kislev doesn't count, because it's rubbish).
How demon summoning works: the first time you summon a demon (pentagram, human sacrifice, true name, funny candles) you make a bargain with it. This involves losing one point of toughness (representing part of your soul maybe?). You can then summon it easily in future, and it will do what you want. Each new demon means a new bargain.
You can restore lost toughness with drugs (remember the evil lawyer bloke's drugs?).