I would posit that is indeed 1d2 con damage per round maximum. Once you've been afflicted by a poison, additional poisonings do nothing but raise the DC of the save they need to make.
I am fairly positive that you still take the initial damage (though only make one roll per round afterward).
Unfortunately admixtures are not discoveries and do not benefit from the Alchemist's discovery DC formula (10 + 1/2 level + Int).
Yes, the more I think about it, the more I believe it ought to be the spell DC.
The precise bombs tax only works on the fire splash damage of minimum bomb damage.
And as a result, it will protect your allies from the viper admixture:
"...and the other vipers attack up to three creatures that take damage from the splash damage."
If they don't take splash damage, they don't take Vipers. And the admixture also implies a choice is made--I believe you can choose to have the vipers affect fewer than three additional creatures.
If an alchemist with viper were to land 4 bombs in a round (not unreasonable for a 12th level alchemist) and they were indeed auto-poison, would mean up to four targets would take 1d4+int and 1d2 con damage, with a save DC of 21 (approx a 75% chance with an okay fort save at CR 12 to negate it next turn).
No, it would mean up to four targets took 4d4 + 4xInt. You seem to be positing the level 12 Alchemist would have a very low Int of +5, so, that's 4d4 + 20 (30 average). +30 damage to up to four targets is an amazing 4th level extract all by itself (yes, because the vipers make touch attacks, I'm assuming they all hit--Touch AC is a joke at level 12+).
I still believe they could take the initial poison damage repeatedly, but even if not, auto-poisoning them with just base Venomous Snake poison would be a DC: 19, not 21.
Honestly as a DM I would either say it either auto poisoned but kept it's DC 13 or used Spell DC (10 + Spell Level + Int) as the save for the poison.
And I would say spell save for sure.
I agree with the consistency thing, though I wouldn't be surprised to find a spell elsewhere with the poison descriptor not offering a save.
I would. There are 23 spells/extracts in Pathfinder with the Poison descriptor (thank you, Archives of Nethys). Viper Bomb Admixture actually isn't one of them, but, yeah, every single spell with the Poison descriptor involves a Fortitude save except for Pernicious Poison, which doesn't actually poison someone, it makes it harder to save vs. poisons.
It is absolutely unheard of and, in my opinion, ridiculous, to think there would be a poison for which you don't get to make an initial save.