Can't we say "vicinity" ?
There's no reason why that shouldn't be accepted
In that case, what's the difference between situations where you'd use בסביבה versus ones where you'd use באיזור?
Large or big should be acceptable
Does is פארק and חונה interchangeable or is there a difference?
They are not interchangeable at all. They just happen to be homographs. פארק - park, a large public garden; חונה - park, as in to park a car.
