100% Correct, Google doesn't like it. if someone posts the same content on multiple platforms. You must be unique in this way.
Definitely, because Google doesn’t know who owns the content and seen it so many times means it’s a copy
Yes, it is. If you still want to publish in different platforms it is advisable to rewrite it in different version without compromising the quality and the information.
It could, as algorithms just scan the content on different places on the internet, and if they see some match ups they can consider it as plagiarism, as they don't know that you wrote the content and just placed it on various channels, so when you do that, try always to modify it a bit or at least change the wording order.