I've never encountered apices or apexes, but the first seems consistent with index/indices or codex/codices, both of which are in use; whereas the second doesn't. Music uses the term coda for something which might more correctly be spelt codex, and the term codicil - meaning a specific addition to an existing body of documentation - survives as a legal term with no equivalents.
Indexes I've never heard used; it sounds quite dreadful and has the disadvantage of not actually being particularly pronounceable. I would always use
stadia and
consortia, if only from habit
Then again, English is notoriously inconsistent in its use of plural forms. House/houses vs mouse/mice is the best-known but there are many more. Some - such as dwarf/dwarfs or dwarves - seem to be accepted either way, and not so long ago the
-oon or
-en form was common in such words as
hosen or
shoon, but you never see or hear it now.
collective terms vary hugely; some bear no visible resemblance to the singular noun - a gaggle of geese; a flock of birds; but some have become words in their own right, such as
military which was once used in the same manner as the still-occasionally-heard
soldieryOED has always included forms that come into use over time. Shakespeare wrote at a time when orthography was more a matter of opinion than fact - there is no record of his name, written by himself in its generally accepted modern form - and probably wouldn't recognise the modern form of his work, Mallory wouldn't be able to understand the modern form of
Morte d'Arthur for all that it has, allegedly, never been out of print.
The King James Bible is readily recognisable but clearly not modern, and again differs considerably from what was actually written at the time.
Then again, there are words of no known etymology which are variously credited to whatever source seems expedient or desirable.
Emmett was generally attributed at one time, to being a Cornish word meaning "ant" and applied to holidaymakers. A little research reveals that it has several possible Old English or Old German origins and meanings, but has no identifiable Celtic etymology.
Grockle is even more obscure.
There are words like
combe, which in spelling and pronounciation is clearly derived from the same root as the modern Welsh word
cwm. There are words like
guarantee and
warranty, which are variants of the same older word which have remained side-by-side with much the same meaning. There are bizarre neo-Welshisms like
egsosts and
siocs, seen lettered on ATS and Kwikfit forecourts in some parts of the Principality.
I think that on the whole, the only useful conclusion on the matter was expressed by the late, great Dr Johnson, who when not occupied berating anyone originating North of Nottingham, entered the following in his masterwork;
Lexicographer: a writer of dictionaries, a harmless drudge, that busies himself in tracing the original, and detailing the signification of wordssignification does not appear in the OED....
''the stopes soared beyond the range of our caplamps' - David Bick...... How times change .... oh, I don't know, I've still got a lamp like that.