What is the difference between a preposition and a word used as if it were a preposition?

Recently two commenters on an online article got into a disagreement about whether “more [adj.] than him” or “more [adj.] than he” is correct to the exclusion of the other. I weighed in with “Both Egg Regis and Squid Viscous [not their real pseudonyms] are mistaken; than can be either a conj. (‘more [adj.] than he’) or a prep. (‘more [adj] than him’).”

To me this is intuitive, but who cares about my intuition? But fear not--Merriam-Webster’s Collegiate Dictionary, 11th ed., agrees with me. Here is MW’s usage note for ²than (prep.) Material in square brackets appears in the first hard-copy printing (FHCP) (2003) and not in the online dictionary (OD) as of June 16, 2011; material in curly brackets appears in the OD and not in the FHCP, and yes, I already know that I need to get a life. Anyhow, here’s that usage note.

After [about] 200 years of innocent if occasional use, the preposition than was called into question by 18th century grammarians. Some 200 years of elaborate [and sometimes tortuous] reasoning have led to these present-day inconsistent conclusions: than whom is standard but clumsy [<Beelzebub...than whom, Satan except, none higher sat — John Milton>] <T. S. Eliot, than whom nobody could have been more insularly English — Anthony Burgess>; than me may be acceptable in speech <a man no mightier than thyself or me — {Shakespeare} [Shak.]> <why should a man be better than me because he's richer than me — William Faulkner, in a talk to students>; than followed by a third-person objective pronoun (her, him, them) is {usually} [usu.] frowned upon. Surveyed opinion tends to agree with these conclusions. Our evidence shows that than {is used as a conjunction more commonly than as a} [is more common than the] preposition, that than whom is chiefly limited to writing, and that me is more common after the preposition than the third-person objective pronouns. {In short,} [You have the same choice Shakespeare had;] you can use than either as a conjunction or as a preposition.

Hear hear!

But that’s not the point of this post. The point is the very strange treatment of this question in The Oxford English Dictionary. (As I’ve mentioned in the past, I refer to the OED without italics, based on The Chicago Manual of Style, which I also refer to without italics: “Names of scriptures and other highly revered works are capitalized but not usually italicized” [CMS, 16th ed., 8.102].) The OED doesn’t recognize than as a preposition, but this is its definition 1b of than (conj.).

With a personal or relative pronoun in the objective case instead of the nominative (as if than were a preposition). This is app. the invariable construction in the case of than whom, which is universally accepted instead of than who. With the personal pronouns it is now considered incorrect.

This strikes me as very odd. “As if than were a preposition”? From a lexicographer’s point of view, what is the difference between a preposition and a word used as if it were a preposition?

That’s the question that all this hoohah was leading up to. And here are a few loose ends.

Why did the exemplary quotation from Milton disappear from the online version of MW’s usage note? The answer is suggested by the OED, which also employs that quotation when discussing than when it’s used as if it were a preposition.

1667 Milton Paradise Lost ii. 299 Bëëlzebub...then whom, Satan except, none higher sat.

From Middle English through the seventeenth century, than was sometimes spelled then. I’m guessing that MW deleted the quotation because using the correct (viz., Milton's) spelling would have just confused the issue. And I like the look of the two consecutive diaereses, but that's beside the point, and I don't do digressions.

It’s worth mentioning, in my possibly worthless opinion, that the OED also lists an obsolete pronoun than.

After a prep.: That; as in for þan for that (reason), therefore; for al þan, for all that (FOR prep. 23b); not (na) for than, notwithstanding that. See also for þan.

I find this stuff interesting, but there's another oddity, probably a computer glitch. "See also for þan" is a link, but it goes to the entry for than as an obsolete pronoun--the entry in which the link itself appears.