The prefix ab-, in some verbs, can denote abnormality or wrongness, not unlike English mis-: e.g. abutor "misuse, abuse". This seems to be the sense of ab-orior, something like "be misborn" (since orior can be used as a synonym for nascor). L&S actually list aborior in their entry for ab -- scroll down to III.A describing the "...
This is just to provide evidence for cōs-cōtis from classical poets:
saepe etiam duris errando in cotibus alas
(Vergilius, Georgica, 4.203)
(Horatius, Carmina 2.8.16)
nil tanti est. Ergo fungar vice cotis, acutum
(Horatius, Ars Poetica, 304)
All of these scan right if and only if cot- is read with a long vowel.
These are all the examples of ...
The original sense of aborior was probably "die", with "be miscarried/aborted" and "miscarry/abort" as derived sense.
Orior originally meant "rise" but took on meanings of "appear" and "be born".
The prefix ab- is used here to mark absence or deprivation.
I think aborior is comparable to the way ...
Below you can see the vowel lengths marked by L&S and by OLD. Note that OLD doesn't cover post-Classical vocabulary.
(In this table L&S = the online L&S via Perseus; OLD = the 1st edition of the OLD consulted by hand; Gaffiot = the 2016 online edition via Logeion.uchicago.edu.)
Vowel in penultimate syllable