There is no such thing as an immortal merc. No matter what gear he wears, you will still have to feed him the occasional potion. And it really isn't a big deal to press two buttons simultaniously - you just need to incoporate it in your playstyle, and you'll be fine.
That has happened one maybe two times out of 200 travincial runs in total.
Chance that Toorc/Ismail/Geleb spawns with Aura Enchanted = (1 - (10/11 * 9/10)) = 2/11 (about 18.2%)
Chance that 1 or more of the 3 spawns with Aura Enchanted = (1 -(9/11)^3) = ~45.2%
Chance that the Aura Enchanted spawn is conviction = 45.2/7 = ~6.5%
Numbers are a bit low because what I calculated in step 2 are the odds of 1 or more of the council spawning with Aura Enchanted. If more than one has an aura, the odds in step two are different (but higher). You should see conviction roughly once every 15.4 visits to Travincal. This ignores any regular uniques in the area that can also spawn conviction.
My numbers might be off a bit - I don't remember seeing Toorc with Aura Enchanted. If he can't spawn that way, the numbers then work out to 33% chance for aura enchanted and 4.7% chance for conviction (1 in 21 trips).
If you've only seen conviction once or twice in 200 trips, you're either very lucky or you didn't see it. Out of 200 trips one expects it to happen at least 10 times.
Curious what level your merc is? Is it an act 2 merc? I'm using an act 2 merc. I've used both holy freeze and defiance and I've used both fortitude and treachery. Always with Andariel's Visage and always with a thresher Infinity. I'm level 96 but since about 85 + I don't think my merc ever dies from the council in travincal. And I don't feed him pots.
As we are speaking I'm 87 - but check your mercs HP, because with your unique based build it gives more +skill points to BO, increasing your mercs life, I wear full-tals so i only get +skill points to BO from spirit and sojs.