when to use "to" vs "in" with elope?

Solution 1:

Merriam-Webster has an interesting article: The Changing Meaning of 'Elope'

Elope appears to have become shorthand for "small destination wedding," "wedding that is not financially insane," or "wedding that allows us to not invite all the people we would rather not invite."

So, using the example you gave:

• "They eloped in New York" = "They had a small, private wedding in the city of New York."

This basically uses the second meaning of Elope, which is less to do with running away to get married without parental consent:

: to slip away : ESCAPE (Merriam-Webster)