Re: Mangni vs Nikah
Mangni can be broken. Nikah requires an actual talaaq to break it.
If you both want that there should be no allowance for changing situation/feelings due to time and distance, do nikah -- but remember: you're married now.
If you want to announce your serious intent to marry, but want to make sure you can survive time and distance first, do mangni -- but remember: gossipers and meddlesome aunties can mess things up while the other person is away. And remember also that living abroad will change a person. The person remaining at home will not necessarily change with the traveller.