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.

“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.