Hi,
On the other hand, Goat Magic is kosher*, but Hog Magic not so much.
I think what you said about everything Infernal is dead on: It is supposed to lose. In this game, anyway.
Anyway,
*When inspected, slaughtered, prepared and consumed according to pertinent Judaic laws. 
One thing I would do is seek a reputation as a Venator with Chthonic Magic. That gives you a certain stigma but also means it'll be much harder to accuse you of diabolism without really solid evidence, and allows you to explain away much of what you do (especially to magi without much Infernal Lore).
Now, actually using the Ars Goetia (including Summoning if learned as an Infernal power) is best done in your sanctum or in extremis. As ezzelino said, having infernal powers is technically not against the Code, but depending on the saga and the Quaesitores, that's really not all that likely to matter - and remember that accused diabolists generally don't get a full trial before the Tribunal, they're usually just Marched without trial by the Quaesitor, which will be ratified after the fact. Trying to lawyer your way out of this one is thus not a good solution - conceal if possible, ally with anyone who sees you if not, and destroy anyone you can't ally with or who gets ideas about blackmail.
Of course, "losing" for the infernal just means "losing" the war for the souls of humanity. An example of such a defeat would be an infernalist who destroys a pious maga through his mighty, hell-given powers ... but shaken by the dying words of his victim ("I forgive you. I have always loved you, and love you still") eventually repents of all his evil on his own deathbed a century later.
Hi,
I mean something different. The Infernal has fundamentally already lost. Good triumphs, because evil is fundamentally stupid in the same way that Wyle E Coyote is fundamentally stupid. The infernal have lost regardless of whether they corrupt souls or fail. They are dangerous powerful but pathetic. They exist to be opposed, and can cause damage yet will always be defeated by resolute opposition. They fail the Evil Overlord Test, all of the tests, because they are the archetypal Evil Overlords. Within the game, they exist for the same reasons they exist in God's Plan: To be abjured, to inspire faith in something better, to (for some monotheists) justify extra merit from doing worthy deeds in the face of adversity, to show the power and grace of God.
Anyway,
Ken
Nah. The Infernal can win any of the battles. It's only the War that's foreordained. Likewise, an Infernalist human is only doomed in the afterlife, not in this life. In this life, he's got cool powers almost for free.
If an infernalist confesses on his deathbed and receives absolution, then he's beaten the Infernal. If an infernalist is martyred for God, he's beaten the Infernal. If he doesn't, then the Infernal has won, both his soul and any souls who his deeds have led to corruption. In the meantime, he can win any of the stories he's involved in.
