thank you hostasaurus for validating my question with your direct answer:
"the migration manager can't use SSH keys for some unknown reason;"
My repeated attempts have verified this conclusion....
Also, it seems the forum administrators don't acknowledge the shortfalls of the plesk system in a professional fashion (with a simple direct answer)... rather, they waste several days of answer/question time with their intimations that the questions are either silly, based on the user's ignorance, or just not worth their time to research...
I like the plesk interface and prefer to use this product and will continue to use the product...I also realize that there is no product in the realm of web hosting that handles anywhere near all of the complex and ever changing situations that may be encountered...
So, please, forum admins and users, take this to heart: a quick decisive answer, even if it discloses a shortfall of the product, is far more valuable to us than multiple days of run-around that implies the question is unfounded...