Betta Fish: New tetras— did they nip his fin off??!

Hi everyone, I would be very appreciative of for advice that will help me take action quickly, especially if the tetras need to be removed.

This is my beloved Mushu. Three days ago I added 10 neon tetras to his tank because I thought it might be nice for him to have some friends. Went to feed him this morning and was horrified to discover he’s missing an entire fin section.

This is the first time I’ve tried to introduce tankmates for a betta and my research seemed to suggest that tetras would be most easily compatible. I was watching carefully for fin nipping or Mushu attacking the tetras. He seemed very chill about the addition, but maybe hiding more than usual. I got a vague sense he might be annoyed by them, and was reconsidering if this would be ultimately beneficial for his well being, but I didn’t witness any dangerous interactions.

The water quality is testing perfect, however he’s on the mend from a mild case of fin rot. Several weeks ago I was out of town and left him for 2 weeks with my son in a travel tank. My son not change the water as instructed and Mushu came back with a bit of fin rot. I added a closeup picture where you could already see the damaged areas regenerating and forming new webbing. Also at the end is a photo of healthy Mushu for full comparison.

Here’s my questions: (all advice and very ideas welcome)

Does this injury look like it was caused by the tetras?

Could it be a result of the fin rot? (I would be kind of surprised because the initial case was mild and appeared to be healing. This looks like a severe loss of fin tissue overnight)

Could he have torn it on something else in the tank?

Is it possible for Mushu to regrow this lost fin tissue or is his quality of life now permanently altered? (He’s swimming ok but obv working a bit harder to get around)

If the tetras are responsible, how should I handle removing them? I’m thinking I could temporarily put them in the smaller travel tank and try to rehome them or take them back to the fish store. I won’t have time to head back to where I purchased them until the weekend because it’s an hour away. Would they be okay in a 2 gallon holding tank? To keep them long term, I’m not prepared to set up another larger filtered tank just for the tetras.