Can 去 be used as a 介词(Preposition)?
You're right that it's not verb serialization, but 去 is still a verb in this sentence. Instead, 怎么走 is the interrogative adverbial phrase that modifies the V+O 去故宫. Consider the sentence structure of a possible answer to the question:

我 骑车     去 故宫。
S  Adv.     V  O 


The structure needs no explanation. Now replacing the adverbial phrase with an interrogative one, the sentence becomes an interrogative sentence:

我怎么走去故宫？


Invert the sentence for emphasis:

去 故宫 我 怎么走？
V  O   |S  Adv.


In the case of 故宫我怎么去, it's the inversion of 我怎么去故宫, where the interrogative adverbial phrase is simply 怎么 without a verb.

我 怎么 去 故宫？
S  Adv. V  O

故宫 我 怎么 去？
O   |S  Adv. V