Hi
I'm afraid I can't answer that for certain, since I wasn't in this extension development team. I believe having it like that allows connection with a different extensions as well. "Association Manager" can be used as a standalone feature without it's multilanguage part
I'm sorry, but You will need to hire a developer to implement such add-on, we do not support customizations unfortunately.
5-1. Yes, that's what I meant. Default site language is the one all the content should be created for first, so it can be translated from it.
5-3. If You for example make translation from English to Polish, selecting this option will create Polish articles, but the content will not be translated and only copied from original one. "Copy Reference to Target" allows to reuse articles already having their associations. Like for example having a page "Translation will come shortly" or something like that.