It is, of course, correct. This book has sold millions upon millions of copies over the last 80 years. If there had been a grammatical error in it , I think someone might have noticed! Bear in mind that the word 'for' is not a preposition in this sentence. It is not 'for' in the sense of 'for me', requiring an object pronoun. 'For' is a conjunction here, meaning 'as' or 'because'. It's a link between the two parts of the sentence. The subject of the next sentence is the pronoun 'I'. The word 'myself' simply adds emphasis. The sentence is fine.

It is correct. The meaning of "for" here means "because", so it can be followed by "I". I agree that "myself" is perfectly correct here, to emphasise "I". We call it an appositive. May I ask why you thought it could be wrong? This is from a very, very famous book.

It is correct here. The "myself" is added for emphasis, to add stress to the fact that "I" was the one who had been searching. Another way to add the stress might be "I personally," but "I myself" sounds stronger.