Actually, Nathan wasn't available. He and Felicia announced the end of their partnership July 16, 2014. Marissa and Mervin had announced their partnership on June 10.
I agree with VietGirl that 5 years residency in the US is apparently minimal for a successful petition for citizenship for Mervin, and that's already passed for 2022. Things could change, but that sort of change tends to come slowly. Tanith Belbin had already lived in the US for more than 5 years, and even then it took an influential Senator from Michigan to push her citizenship through by an act of Congress, just under the wire in Dec. 2005 for Torino.
I was fascinated by listening to this TSL interview of Marissa and Mervin in 2015. She said she'd already accomplished more, with Simon, than she'd expected. Another Olympics didn't seem to be a priority for her. She wanted to be a world-class pairs skater again, but even that seemed less important to her than being friends and more connected with her partner. She believed she could be that with Mervin. They didn't have that possibility till this year (if "world class" is defined as going to WC) ... and we all know what happened. I don't know, are they "world-class" pairs because they've earned spots at GP events? Would a B competition qualify?
https://www.youtube.com/watch?v=Gl7mcJhS