Can you provide some citations for locative prepositions appearing before their nouns? I am not widely read in classical Chinese, but all the examples I've turned up have used the normal order.

e.g. 孟子 1A/3 河內凶，則移其民於河東，移其粟於河內 (strongly suggesting that the current naming practices were also current 2200 years ago); 庄子 秋水 13 我知之濠上也 (ordinary positioning).

You're totally right, I was confusing modifiers with Chinese's general looseness towards word class, a tendency even more pronounced in Classical Chinese [1]. That means what looks like modifier/preposition could be functioning as a verb instead.

So my mother-in-law just said '上海不在海上', i.e. 海上 is closer to the literal meaning of 'on top of the ocean (physically)', which the city is not.

Perhaps 上 here acts as a verb or gerund ((going up / on the way) to the ocean)? 下海 means 'going (down) to the ocean'.

The rabbit hole here can get deep (and thanks for drawing me in!), this book [2] goes in-depth about how word class and word order influenced one another as Chinese developed.

[1] https://en.wikipedia.org/wiki/Classical_Chinese_grammar#Word...

[2] https://www.google.com/books/edition/_/r9WO6g5mqi8C?hl=en&gb...

> Perhaps 上 here acts as a verb or gerund ((going up / on the way) to the ocean)? 下海 means 'going (down) to the ocean'.

This is possible, but I don't think it's a coincidence that the normal way to express that is 下海. The ocean is pretty much by definition both "down"stream and downhill of everything; approaching the ocean by going up is semantically unnatural. My guess above of "get onto the ocean" (i.e. "set sail") was drawing a parallel to 上路 "get on the road" / "start traveling".

> 海上 is closer to the literal meaning of 'on top of the ocean (physically)', which the city is not.

I don't see this as a huge obstacle; "upon the sea" has the same literal meaning but isn't surprising in an English place name, though "by the sea", with the correct literal meaning, isn't surprising either.