Let's answer your questions one by one, ok?
Well, to give you ideas, Macross 25 is the 55th large-scale, super-long-distance colonization fleet. So there's 54 colonization fleet before Frontier, which 24 of them are New Macross class, and 13 of them are Megaroad class. They usually are self-dependent, and with good reason: with the fold faults, traveling from Earth to another colonization fleet takes way too long the time, although research about fold booster that bypass the faults is being tested by LAI, and used by Micheal to reach Galia 4. No info is given, as far as I recall, if the colonization fleet have constant communication with Earth, although I think they wouldn't do that too often.
Yup.
According to the latest spoilers,
Well, seeing that Galaxy did asked for Frontier's help when their ship is being attacked by Vajra during the early episodes of Frontier, I say yes.
I think his speech is directed to the people of Frontier, and the term humanity is more likely referred to the citizens of Frontier. That, and politicians love use big words to emphasis their point, especially someone like Leon. Don't take what his said literally.
Well, there's one example: Macross 5 has strong Zentradi motives, and possibly designed to carry Zentrandi for colonization project.