If you cleared CTET in 2018 under the OBC-NCL category, that certificate remains valid for eligibility as long as CTET validity rules allow it. However, for KVS PRT recruitment, your category is decided based on your current OBC-NCL status, not the one you had in 2018. Since you mentioned that
Yes, you can choose Mumbai as your CTET exam centre even if you select Assamese as your language paper. The exam centre choice depends on your current location, not your state of residence. However, make sure that Assamese is available as an optional language while filling the CTET form