Hi Ater!

Arunachal Pradesh grants domiciles to the following people: 

• Indian citizens who are not originally from the State but work in the Govt. service.
• Indian citizens who are not originally from the State but have been staying in the state for 5-10 years.

Hence if you fulfill one of the above conditions, you arw eligible for a domicile of Arunachal Pradesh.

With 422 marks in ST category your chances of getting an mbbs seat are pretty high. It is a good score for your Category, hence you needn't be confused. 

Hope this helped!