Single word to replace "allowed to be missing"

The usual mathematical terms for these things (from the study of modal logic) are 'necessary' (for your 'guaranteed') and 'possible' (for your 'allowed'). All you need is negation to get all four possiblities.

necessary - it must exist
possible - it may exist
not necessary - it may not exist
not possible - it cannot exist

Depending on your (choice of) logic those two in the middle may be the same.

For a logic of probabilities, where 0 <= p <= 1:

necessary: p=1
possible: p > 0
not necessary: p < 1
not possible: p = 0

For example, you can see that 'not possible' is the same is the complement of 'possible'.

This mathematical use of these words follows our informal meaning.

So to your specific questions:

Which single word is able to replace the phrase "allowed to be missing?

With respect to probability, this means that it could be any probability. So any combination that covers all possibilities, 'necessary or not necessary'
Which single word is the exact negation of "guaranteed to exist"?

By negation, there are two possibilities that informal English allows. 1) the set complement, 2) the other point extreme of the spectrum.
- For the set complement it is 'not necessary'.
- for the other extreme it is 'not possible'.

This is commonly denoted as optional:

available as a choice but not required

_{(source: Merriam-Webster)}

Another example:

The definition of a method, constructor, indexer, or delegate can specify that its parameters are required or that they are optional. Any call must provide arguments for all required parameters, but can omit arguments for optional parameters.

Something that is allowed to be missing is omissible. Wordreference.com defines omissible as: