I think I can answer all of your questions by saying that from an English viewpoint, the Bible (the English version of which is just a translation) is inconsistent. Whoever has done the translation job has had to decide between singular and plural on each occasion and has not always got it 100% grammatically correct.
Without wishing to be personal to you, if you revere the Bible, you expect it to be perfect because it is "God's word". However, it is not perfect, particularly when translated into English.
I'm afraid you'll just have to be tolerant of some good old-fashioned grammaqtical mistakes. Try running some of the above examples（注：No.15の英文）through a word-processor grammar-checker to see what I mean.
Hope this helps to put things into context!
1. Jesus's voice from heaven underscored the truth which runs through all the prophets' messages -- so the overall message (singular) is to be believed -- not depending on the specifics of each message.
2. your are right -- should be heads as the other 2 common nouns are plural.
3. while there could be some exegetical reason why the flip in detail order and change in number (the flip itself could signal some change in meaning to explain why the first spoke of the collective mind and the second of the personal), this could be an issue of poor translation from the Greek.