In some cases, that's correct. For instance, when the subject is 私, あなた, これ/それ/あれ, "この/その/あの + noun" or like that, は is used for simple statement. Similarly, は is also used when the sentence has a nuance of "generally".Previously I was thinking that は, not が, would be used in simple statements such as that one.
Yeah, that's right. As you know, the emphasis is put on the noun/phrase/clause preceding が and the one following は. So 弟が日本に来たら、私は（彼を）日光に連れて行きたい can also mean as below.OK, so 私が is "I'll be the one to take him" (emphasis on the subject), and 私は "it's Nikko where I'll be taking him" or "it's Nikko where I'll be taking him" (emphasis on either the actor as contrast and the destination or just the destination)? Does that seem right?
Yeah, that's right. As you know, the emphasis is put on the noun/phrase/clause preceding が and the one following は. So 弟が日本に来たら、私は（彼を）日光に連れて行きたい can also mean as below.
Someone may not want to take him to Nikko, but I want.
Someone may want to let him go alone, but I want to take him.
Someone may want to take someone else than my brother(family members, friends, etc.) to Nikko, but I want to take him.
Exactly!I'm assuming then that the context would make the meaning clear?
It's more likely to say "whether the subject or object, two simple sentences or one compound sentence, when it's obvious from the context, the word can be omitted".Also, in the first example,弟 went from being the subject to the object. That throws me a bit, to be honest. Perhaps that's one facet I've been missing this whole time, but I was under the misinterpretation that a subject could not also be an object? Or is this only possible because there are two clauses in those sentences?