Using Al-Iksir to remove Aging Points or Reduce Age does have a significant cost, though. Taking away ten years adds ten Warping Points, or other negative effects to the character. So unless the sahir wants to take that penalty every ten years, the years will eventually catch up to him.
Also, the beneficial modifier to aging rolls is only one-fifth of the sahir's scores in Solomonic Alchemy and Physic. Once he has reached scores of 25 or 30 in each it seems like it's impossible for him to increase them fast enough to offset natural aging, and even then I don't think the bonus to his modifier would be high enough to make a crisis impossible.
What do you mean by "win the summons lottery"?
And more Warping Points, of course. Plus they are gone for a whole year, which means they don't get to study during that time, and also need to work extra seasons when they get back, to make up their lost service to the Suhhar.
That sounds like a fun antagonist, but what naranjs are you thinking of when you say they "don't actually need to penetrate magic resistance being a natural change"? Solomonic Rituals are active magical effects and still need to penetrate, and remember that they use the spirit's Penetration and Finesse scores, not the sahir's. Are you thinking of effects like Summon forth the purest flame?
I really like how sahirs can use demons with their magic without automatically gaining the wrath of the Suhhar (though of course the Divine isn't happy). For the most part, they are judged on what they do with their magic rather than how they do it. It takes a crafty sorcerer to keep a demon doing what he wants it to do, but that's just part of the challenge, right?