Update
According to this post by Rhys, most uniques can actually spawn on any item of the correct base type, regardless of item level:
In general, the required ilevel to drop or chance a unique is the same
as the base type. However, there are now exceptions to this rule, such
as Auxium, Le Heup of All, and Atziri's Foible. These currently
require an ilevel equal to their equip level.
As far as unique Onyx Amulets go, the minimum item level is still 20, as that's the minimum item level for Onyx Amulets in general.
Previous answer
Just as for normal items, the minimum item level of a unique item is the same as the minimum level to equip it. The unique Onyx Amulets all require level 20, just like a normal one, so any Onyx Amulet you find could potentially be chanced into any of the unique ones (bar any exceptions set by GGG to prevent new copies from being obtainable).
It's kind of rare for a unique item to have a different required level from its normal counterpart, but exceptions do exist (e.g. the new Voll's Devotion Agate Amulet is level 32 while a normal Agate's is 16). If there's a particular unique you're interested in, look it up on the Path of Exile wiki.
You can always apply elemental ailments on any hit, whether or not it is a critical strike, dealing a crit will just set that chance to 100%. These gloves take away that 100% chance so you will have to rely on having freeze/shock chance from skills, support gems, the passive tree, weapon modifiers etc.
Best Answer
If you have multiple Unique organs, and you want a specific Unique organ to drop, you should aim for a combination of body parts that will still result in a juiced up metamorph, but only have one Unique part slotted in the desired drop part slot.
Example: Say you have the following:
You are looking for a Unique Liver drop from the metamorph. Therefore you should aim for a combination that not only results in the maximum juice bar, but also looks something like:
This will ensure that you always get a Unique Liver upon defeating the metamorph.
If you set up your metamorph as the following:
As of patch 3.9.1, every map boss has an equal unweighted chance to drop any of the 5 parts, so every boss can drop brains, eye, livers, lungs, and hearts.