Not only but also : complex or compound

“Not only did George buy the house, but he also remodeled it.”

George did buy the house - George is the subject, did buy is the verb [and helping verb I believe], house is the direct object. There is one subject, George. But George did two things. Not only did he buy but he also remodeled .

I'm no expert, but I would definitely not consider the first segment of this sentence to modify remodel. Adverbs modify verbs. There could easily be two separate sentences - both having subjects and verbs, indicating neither is dependent; however, the two actions DO go together in that he did both things. I'd say complex is a good word to use. I Googled complex sentence and read this:

Two clauses connected with a coordinating conjunction is a compound sentence.
Two clauses connected with a subordinating conjunction is a complex sentence.

So I Googled "subordinating conjunction" and found:

Subordinating conjunctions are conjunctions that are used at the beginning of subordinate clauses. Some examples of these conjunctions are; although, after, before, because, how, if, once, since, so that, until, unless, when etc.

"Not only ... but also" is a correlative conjunction. In further research, I found sentence examples as to when a comma is needed. Because the second clause is also a complete sentence, the sentence would be grammatically incorrect without the comma before “but.”

I'm curious to see what other comments are made on this.

To address your concern with interrogative vs declarative, the sentence COULD read as follows:

George not only bought the house, but he also remodeled it.

I can't speak to what constitutes a complex sentence. That is a level of grammar that is above my head. But I will say regarding the interrogative format and the fact that there is a version where you can restate it without that interrogative format, that to me the interrogative format indicates that this is an answer to an actual interrogative. In other words someone asked me the question did George buy the house. Obviously that format has the verb before the noun which is typical in English for an interrogative. I am then answering in a sort of exaggerating way or at least an emphasizing way, hey not only did he do that but he did something more, that nut.

I think you could be right in a loose sense that buying the house is a clause that modifies remodeling the house. It is not an instance of an adverb modifying a verb. Maybe some kind of grammarian's perspective of one clause modifying another. I don't know.

I would say that with the use of "not only", the "also" is simply redundant, and so unnecessary. The "but" might not even be needed, though just being concise for its own sake isn't always the best way to go.

"Not only did George buy the house, but he remodeled it."