First of all the process does not state that it cannot accept its own product as an input. In order for the product not to be a valid input, it would need to specifically exempt it, as it also specifies both that the process works on any poison that comes in doses (not exempted, of course) and that the end product is such a poison.
Next, the ability does state that you need two doses of the same poison. While a case could be made that the base poison and its extra-strong form are the same poison, it seems pretty clear from context and the workings of similar rules that they should not be. So, particularly given your balance concerns, of the two equally RAW interpretations the one differentiating between concentrated and non-concentrated poisons is probably the better choice, especially if Paizo ever publishes a poison variant of the contagion spell.
With outside assistance, there is no maximum limit to the concentration of poison, though the resources required to achieve each level past the soft cap are exponential in growth. Without outside assistance the total time required in minutes is \$2^n - 1\$ where \$n\$ is the desired concentration level of the poison, and the cap is hit when the difference between a level and the previous level is more than 50-60 minutes (depending on GM ruling as to when the dose is used, RAW seems to slightly favor the shorter time limit. We will see later it turns out not to matter, though). This makes the rule:
$$(2^n-1)-(2^{n-1}-1) \le 50$$
or
$$2^n-2^{n-1} \le 50$$
or
$$2^{n-1} \le 50$$
so the cap is at 6, since the 7th level would require 64 minutes of preparation to make, and the 6th level only requires 32. Conveniently, this eliminates any need to resolve our previous ambiguity as to when, exactly, a dose is used.
The 6th level of concentration results in a poison that costs 64 times as much, takes 63 minutes to make and is spoiled by even the slightest interruption, lasts for only an hour itself, and provides a 1039% increase to duration (~11× as long) and +12 to the save DC. This is a fairly expensive option for the meager power it provides, but certainly goes a long way towards making poisons of the less combat oriented type much more viable when used (since you will generally have time to prepare them safely).
Ok, so, for curiosity, I've been trying to optimize the solo crafting order to maximize the highest level of potion created. This is pretty much just a math problem; don't continue reading this unless you are interested:
My best so far is:
first hour: get 60 level 1 poisons from 120 level 0 poisons (regular doses)
second hour: get 30 level 2 poisons from 60 level 1 poisons; get 30 level 1 poisons
third hour: get 16 level 3 poisons from 32 level 2 poisons (two of which were crafted at the end of the cycle using produced level 1 poisons), get 30 level 2 poisons from 60 level one poisons (leaving us with 28 left for next time), get 14 level 1 poisons
fourth hour: get 8 level 4 poisons from 16 level 3 poisons, get 14 level 3 poisons, 8 level 2 (one produced from lv 1 poison from this turn), 30 level 1 (28 at end of turn)
fifth hour: 4 level 5 poisons, 8 level 4 poisons (1 produced from threes made this turn), 4 level 3 poisons (leaving 2 for next turn), 15 level 2 poisons, 28 level 1 poisons
sixth hour: 2 level 6 poisons, 4 level 5 poisons, 2 level 4 (1 using poisons produced this turn), 8 level 3 (1 using poisons produced this turn, leaving 6 at end of turn), 14 level 2 (leaving 12 at end of turn), 30 level 1 poisons
seventh hour: 2 level 7 poisons (1 using poisons produced this turn), 2 level 6 poisons (0 at end of turn), 2 level 5 poison, 3 level 4 poisons (1 at end of turn), 6 level 3, 15 level 2, 30 level 1 poisons
eighth hour: 1 level 8 poison, 0 level 7 poison, 1 level 6 poison, 1 level 5 poison (from 1 level 4 produced this turn), 3 level 4 poisons (2 at end of turn), 8 level 3 (1 from 1 level 2 produced this turn, 15 level 2 (14 at end of turn), 30 level 1 poisons.
Sometimes
From the SRD, italics mine:
...when an attacker that you can see hits you with an Attack, you can use your reaction to halve the attack's damage against you
In other words: Uncanny Dodge works only against the damage of the attack itself. Effects that add directly to the damage of the attack - such as Sneak Attack - would be affected, but secondary effects requiring saves would not.
@Slagmoth has pointed out that this is backed up by an answer in an official rules supplement: the Sage Advice Compendium. The relevant ruling, italics mine:
Does Uncanny Dodge work automatically against every
attack a rogue or ranger gets hit by? Spell attacks too?
A use of Uncanny Dodge works against only one attack,
since it expends your reaction, and only if you can see the
attacker. It works against attacks of all sorts, including spell
attacks, but it is no help against a spell or other effect, such
as fireball, that delivers its damage through a saving throw
rather than an attack roll.
In the examples given:
- The poison on the weapon is a secondary effect requiring a save, Uncanny Dodge does not apply
- The basilisk's venom is part of the attack, Uncanny Dodge halves the damage taken
- The poison damage from the wyvern's sting is a secondary effect requiring a save, Uncanny Dodge does not apply
Best Answer
Following the precedent set forth by the Sage Advice Compendium, "immunity to poison" includes both the Poisoned condition and the poison damage type
The Sage Advice Compendium has two instances of explaining what "poison" means:
We note that both of these features use the same wording as Nature's Ward:
Thus there is no reason to believe that Nature's Ward would work any differently. "Immunity to poison" means you are immune to both the Poisoned condition and poison-type damage.