It is unlikely that the inertial mechanism is damaged from your description . The reel
has to be vertical or it will not release. If the belt is tight and you cant get any slack to release the belt then undo the top bolt and get some slack. . Be patience and gentle and you should get the reel released.
If you do have to replace it I have a spare
however having removed a few of these belts the inertial reel mechanism is not designed for user access / repair and to be honest if you need to reach that stage would you trust your life to it or one of your kids etc... I wouldnt.