My experience with a malnutritioned mandarin is that once you get them eating SOMETHING after awhile try rotating back in all the frozen stuff one at a time that they did not like. I experienced a mandarin that wouldn't eat hardly anything untill I got it to eat one type of food. After a week it started taking other stuff and pecking at rocks again here an there. I think possibly when they get that skinny they are sick and really don't eat or act properly. I'd start with frozen brine or baby brine. Blood worms are popular with one of mine.
I've seen very skinny mandarin live a good while. Once it starts eating it will take a good while for it to look healthy looking. I'm not talking a week or two. You will notice weight gain starting at the pectoral fins. If you see even a smidge of weight gain after a week of them eating then you should be ok.
If your able to buy live brine then I would def mix in frozen. thaw it in a net under warm water before you dump it in. If I was in your situation I'd keep the mandarin in the QT until you get it eating something frozen if you tank cannon support it pod wise. For that matter I have a mandarin that doesn't even eat pods. If your mandarin is pecking at invisible stuff then its eating pods.
If at all possible feed it multiple times a day please. especially with a very malnutritioned mandarin its important to allow it to feed often. They don't typically take more than a bite or two of adult brine. Since you have live brine you are able to buy that is ideal since they can snack as they will.
The problem with mandarins is I can with fair certainty that if a mandarin has no other food source in the tank. Getting fed one time per day will either just barely maintain..or more likely lose weight unless there is uneated ammounts of its perfered frozen food that will sit at the bottom for an hour+ and not get eaten by other fish.
Sorry for the length bits of scattered info. I just wanted to spew out as much as I can for you. I own 3 mandarins and a scooter blenny.
Do you have a good picture of your mandarin?
Here is a link to more info i've blurted out in the past:)
https://www.livingreefs.com/my-experience-mandarins-and-particularly-picky-new-addition-t26373.html