I didn't read any of it but heck... it's gonna be a chore I bet.
I recently switched from windows 10 mobile to android and all my contacts strangely disappeared in groups starting after a couple of days of use, even though I had all my contacts stored on google contacts.
If you mean transferring the entire phone (like when you set-up a new phone) back and forth between the two then I just use the built in transfer tools. Has worked flawlessly with iPhone, Google, and Samsung phones for me.