Not a pro on this kind of thing, but yes, you just use the phone or click on the sim's portrait in the relationship panel. Also, you will not be able to control them, but if you ask them to marry right after they got there then you can. And yes, I've had my sims marry while one is visiting from a foreign place.
Side note: make sure you invite the sim in immediately when they arrive or else they will leave. Very annoying, you would have to invite them again.
Hope that was helpful!