Modifying my knowledge of language always, I remain open-minded to how the rules of our language are in flux. Those rules and their specific relationship to lexicographical changes in partciular also challenge me to remain open-minded to this language in flux. To fight the changes would be to turn into a stone.

Now prepositional crossovers are what I think of as words that weren't always prepositions, but at some point in the history of the language became recognized as functioning recognizably and immediately as a preposition functions.

Consider the following list of simple prepositions:

from
for
off
of
to
in

...and so on for the simple ones.

Now consider a list of compound prepositions:

alongside of
along with
out of
outside of

...and so on.

But the most interesting group to me are those present participle forms--those verbs with the 'ing' attached--that somehow over time developed into being both present participles and prepositions:

concerning
considering
regarding
excepting

...and so on.

My question is:

How do the lexicographers determine at what point a participle may also receive a nomination for becoming a preposition?