Here you can find the exact stats for Juggernaut and Terminator
No, since the turret that Juggernaut is equipped with is "Terminator" and has nothing to do with Railgun nor Striker.

If you remember the nickname correctly, you can either try sending a gift and write on it your social media informations (facebook profile etc...) . In case the player has gifts turned off or you just can't send a gift for whatever reason, you can then send a private message in forum. If you don't know how to do so, this guide should help you.
 
Or just a simple thing: Send an in-game friends request and whenever that person is online, text them ?

Log in from HTML version, go to Mission then Special tab, there you'll find the premium rewards and all you have to do to claim them is press on Collect
 

It is due to the migration, as far as I know it's a known bug and being investigated as we speak.
Although it's just a visual bug, players can still join battles according to their ranks limits

As one of the forum moderators told you, your problem was forwarded and is being investigated, a little bit of patience please ?

If you want to become a Tester, apply here