I will try to keep my answer very short.
Firstly, this is an epic story and fictional, the characters play the writers song. So yea, Gandalf’s plans are unknown because Tolkien, as far as I know, never put down all his alternative ideas for the Lord of the rings, like the plan for the Community of the ring. That will be kept a mystery forever unless his son, Christopher discovers some papers in which appear some possible ideas for the Community’s roads.
Secondly, yea if I were Gandalf, I would have also planned something like the eagle plan. I would have asked Radagast, when Saruman uses him to lure Gandalf into Orthanc, to send some friends “eagles” to go around spying the Enemy and also to get them ready and prepared to come into Lorien, when the time comes, or having at least one eagle messenger “Gwairhir” flying between Lorien and the eagles nests. If you remember after defeating the Balrog at the top of Zirak-Zigil Gandalf arrives into Lorien with Gwairhir’s help.
So yea the eagle plan theory basics are totally possible and are correct for me. But don’t use the wrong ideas from the films “butterfly”, like Gandalf casting the eagle spell so freely and cheaply.
And lastly, we should already know that the eagles always appear in key points around Gandalf, when he is in total peril and nothing else could have been written that would have either way broken the logic of the third age marc. I don’t wanted to see for example a Valar saving the day, although the eagles are like sons of Manwë.
Tolkien put the eagles and their home on the Misty Mountains, that’s all.