In is a preposition, In that place is not. It is a prepositional phrase that is acting adverbially.
Even you
identify the phrase as prepositional phrase and state that it is
acting as an adverb. Let's say I accept that it [the
phrase] is acting as an adverb. Fine. Now you tell me, suppose you write the "Phrase a Day" email and today's phrase is "in the place." Do you identify this phrase as an adverb? Would that be the most direct way of identifying what this phrase is? I say that no, it would not. This "Phrase of the Day" is a prepositional phrase. That's what it is and what it will remain when it punches the clock and goes home to the wife. If you want to construct a sentence that uses it as an adverb, go right ahead. But by and large this phrase is a prepositional phrase, and if you intend to introduce it to the world it would be most appropriate to introduce it as what it really is, not as what it is currently serving as in a particular case.
Similarly, if you're hosting a cook-out, and the neurosurgeon next door is helping you out by cooking the hotdogs, you might playfully introduce him or her as your chef, but if someone asked you in seriousness what he did for a living, you wouldn't say "Oh, he's a hotdog chef" except as a jest. In seriousness, you'd say he was a neurosurgeon.
Or, if you know of a famous movie director who uses an Academy Award statue as a door stop, you wouldn't identify such a statue as a doorstop in a didactic setting. You'd identify it as an award.
So now let's turn back to this phrase borrowed from Latin,
in situ. How would one best introduce this phrase to the world? As an adverb? I say no. That's not the
essence of its being. It is a Latin prepositional phrase that that can be pressed into service, I will grudgingly admit, as an adverb in a very broad sense, but it is very often used as an adjective. So in putting together a list of words or phrases under the heading of adverbs, I would not include this phrase.
