For Kerala, domicile  eligibility is that the person should have been the resudenve of Kerala for more than 10 years and for Maharashtra it is 15 years residence criteria. 

Hence, according to me you have to claim reservation of any of the above two state you have to fulfil the requirdr criteria 

No you cannot get domicile certificate of Maharshtra

Babita you need to clear the eligibility test for getting a domicile of Mumbai plus class X and XII should also be done from Maharashtra to be eligible for getting state reseravtion quota.

Atleast a stay of 15 year is necessary when a person can claim state domicile of Maharashtra.

Thank you