Reputable installers are either HIES registered or provide an insurance backed guarantee (IBG), which can protect you if an installer goes bust and a guarantee which normally lasts 2yrs. An IBG certificate can be sent by the insurance provider or handed over by the installer with all other paperwork on completion.
However, its no good crying over spilt milk if neither of these apply, Its still possible to a MPAN reference from your DNO to provide to you nergy provider, which you have done and now covers export. Unfortunately any costs to rectify faulty workmanship, will need to be met by you and a warranty claim can be made by you or if the new installer is willing to the manufacturer or supplier in the event of faulty equipment.
The MCS is a typical shambolic government back scheme, who offer you no protection should things go wrong. Scotland energy trust seem no better and given you require a MCS certificate in Scotland its poor show,
That said, now the DNO have given you a MPAN ref, I wouldn’t fret too much about a cert at this point.