What is the purpose of the first session?
The adventure you're suggesting will introduce the basic combat and character rules, the Infernal but in an ubsubtle plot that does not do justice to its corrupting yet virtue-free nature in RAW Ars (whether you want to adopt this view is a different question), and then a taste of formulaic magic. Is this really what you want to open with? First impressions count, and I stand by my suggestion to open with the very best ArM has to offer - whatever you think that is.
I suggest deciding first what you think will be the coolest thing about playing your ArM campaign, and starting with something along this theme. From what you've written of it, it seems you are excited about subtle plot of hidden villains against a confused, highly-magical backdrop. So do something like that, in miniature. To give them a taste of the entire campaign, if you will.
A classic start is Nigrasaxa - that showcases Twilight, magic use, and covenfolk/grogs. Why not change it to suit your theme? Instead of Twilight, have the dead magus botch a deal with the Infernal. Replace the lightning with minor demons. Have the entire covenant sink into an Infernal aura. Getting to the wizard's lab will allow the magi to solve the aura problem, but the demons will persist; perhaps the mage bargained for immortality, and was granted his wish in the form of eternal Infernal torment, sending the covenant to Hell and leaving him paralyzed with pain - so the curse will be lifted when he is killed. Which, at this point, he'll be begging for - which will work for the demon, too, since this earns him his soul. The next session will be devoted to politics, internal and external, as the PCs try to control the covenfolk and deal with the impending Quaesitorial visit. This kind of story, while far simpler and more brute, highlights that the Infernal can be patient and clever and pit one magus against the other, will showcase strange confusing environments and highly magical traps in the wizard's sanctum, and allows room for having PC magi as well as PC grogs (by switching point of view/characters) to showcase both levels of play.
Of course, since this is my adventure-building it probably won't suit your tastes. But I suggest sticking to what gets you excited about the game - hopefully, your excitement will rub off on the players and session. This is especially important in a first session with a new party of gamers and a new game system, IMO.